SHKD

Посмотрите https://bigmoll.com/ она проще
Для opencart-3
нужно catalog\controller\common\home.php заменить весь код на этот

<?php
	
	class ControllerCommonHome extends Controller {
		public function index() {
			
			
			if ((isset($_SERVER['HTTPS']) && (($_SERVER['HTTPS'] == 'on') || ($_SERVER['HTTPS'] == '1'))) || $_SERVER['SERVER_PORT'] == 443) {
				define('HTTP',"https://");
				} elseif (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' || !empty($_SERVER['HTTP_X_FORWARDED_SSL']) && $_SERVER['HTTP_X_FORWARDED_SSL'] == 'on') {
				define('HTTP',"https://");
				} else {
				define('HTTP',"http://");
			}	
			
			define('HOST', HTTP.$_SERVER['HTTP_HOST']);	
			
			
			
			$this->document->setTitle($this->config->get('config_meta_title'));
			$this->document->setDescription($this->config->get('config_meta_description'));
			$this->document->setKeywords($this->config->get('config_meta_keyword'));
			
			
			
			$this->load->language('product/category');
			
			$this->load->model('catalog/category');
			
			$this->load->model('catalog/product');
			
			$this->load->model('tool/image');
			
			if (isset($this->request->get['filter'])) {
				$filter = $this->request->get['filter'];
				} else {
				$filter = '';
			}
			
			if (isset($this->request->get['sort'])) {
				$sort = $this->request->get['sort'];
				} else {
				$sort = 'p.sort_order';
			}
			
			if (isset($this->request->get['order'])) {
				$order = $this->request->get['order'];
				} else {
				$order = 'ASC';
			}
			
			if (isset($this->request->get['page'])) {
				$page = $this->request->get['page'];
				} else {
				$page = 1;
			}
			
			if (isset($this->request->get['limit'])) {
				$limit = (int)$this->request->get['limit'];
				} else {
				$limit = $this->config->get('theme_' . $this->config->get('config_theme') . '_product_limit');
			}
			
			
			$data['compare'] = $this->url->link('product/compare');
			
			
			function str_replace_once($search, $replace, $text) 
			{ 
				$pos = strpos($text, $search); 
				return $pos!==false ? substr_replace($text, $replace, $pos, strlen($search)) : $text; 
			}			
			

			$data['products'] = array();
			
			$filter_data = array(
			'filter_category_id' => 0,
			'filter_filter'      => $filter,
			'sort'               => $sort,
			'order'              => $order,
			'start'              => ($page - 1) * $limit,
			'limit'              => $limit
			);
			
			$product_total = $this->model_catalog_product->getTotalProducts($filter_data);
			
			$results = $this->model_catalog_product->getProducts($filter_data);
			
			foreach ($results as $result) {
				if ($result['image']) {
					$image = $this->model_tool_image->resize($result['image'], $this->config->get('theme_' . $this->config->get('config_theme') . '_image_product_width'), $this->config->get('theme_' . $this->config->get('config_theme') . '_image_product_height'));
					} else {
					$image = $this->model_tool_image->resize('placeholder.png', $this->config->get('theme_' . $this->config->get('config_theme') . '_image_product_width'), $this->config->get('theme_' . $this->config->get('config_theme') . '_image_product_height'));
				}
				
				if ($this->customer->isLogged() || !$this->config->get('config_customer_price')) {
					$price = $this->currency->format($this->tax->calculate($result['price'], $result['tax_class_id'], $this->config->get('config_tax')), $this->session->data['currency']);
					} else {
					$price = false;
				}
				
				if ((float)$result['special']) {
					$special = $this->currency->format($this->tax->calculate($result['special'], $result['tax_class_id'], $this->config->get('config_tax')), $this->session->data['currency']);
					} else {
					$special = false;
				}
				
				if ($this->config->get('config_tax')) {
					$tax = $this->currency->format((float)$result['special'] ? $result['special'] : $result['price'], $this->session->data['currency']);
					} else {
					$tax = false;
				}
				
				if ($this->config->get('config_review_status')) {
					$rating = (int)$result['rating'];
					} else {
					$rating = false;
				}
				
				$data['products'][] = array(
				'product_id'  => $result['product_id'],
				'thumb'       => $image,
				'name'        => $result['name'],
				'description' => utf8_substr(trim(strip_tags(html_entity_decode($result['description'], ENT_QUOTES, 'UTF-8'))), 0, $this->config->get('theme_' . $this->config->get('config_theme') . '_product_description_length')) . '..',
				'price'       => $price,
				'special'     => $special,
				'tax'         => $tax,
				'minimum'     => $result['minimum'] > 0 ? $result['minimum'] : 1,
				'rating'      => $result['rating'],
				'href'        =>   $this->url->link('product/product', 'path=' . null . '&product_id=' . $result['product_id'] )
				);
			}
			
			
			
			$url = '';
			
			if (isset($this->request->get['filter'])) {
				$url .= '&filter=' . $this->request->get['filter'];
			}
			
			if (isset($this->request->get['limit'])) {
				$url .= '&limit=' . $this->request->get['limit'];
			}
			
			
			$data['sorts'] = array();
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_default'),
			'value' => 'p.sort_order-ASC',
			'href'  => HOST . ('/?sort=p.sort_order&order=ASC' . $url)
			);
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_name_asc'),
			'value' => 'pd.name-ASC',
			'href'  => HOST . ('/?sort=pd.name&order=ASC' . $url)
			);
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_name_desc'),
			'value' => 'pd.name-DESC',
			'href'  => HOST . ('/?sort=pd.name&order=DESC' . $url)
			);
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_price_asc'),
			'value' => 'p.price-ASC',
			'href'  =>HOST . ('/?sort=p.price&order=ASC' . $url)
			);
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_price_desc'),
			'value' => 'p.price-DESC',
			'href'  => HOST . ('/?sort=p.price&order=DESC' . $url)
			);
			
			if ($this->config->get('config_review_status')) {
				$data['sorts'][] = array(
				'text'  => $this->language->get('text_rating_desc'),
				'value' => 'rating-DESC',
				'href'  => HOST . ('/?sort=rating&order=DESC' . $url)
				);
				
				$data['sorts'][] = array(
				'text'  => $this->language->get('text_rating_asc'),
				'value' => 'rating-ASC',
				'href'  => HOST . ('/?sort=rating&order=ASC' . $url)
				);
			}
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_model_asc'),
			'value' => 'p.model-ASC',
			'href'  => HOST . ('/?sort=p.model&order=ASC' . $url)
			);
			
			$data['sorts'][] = array(
			'text'  => $this->language->get('text_model_desc'),
			'value' => 'p.model-DESC',
			'href'  => HOST .('/?sort=p.model&order=DESC' . $url)
			);
			
			

			
			$data['limits'] = array();
			
			$limits = array_unique(array($this->config->get('theme_' . $this->config->get('config_theme') . '_product_limit'), 25, 50, 75, 100));
			
			sort($limits);
			
			
			$url = '';
			
			if (isset($this->request->get['filter'])) {
				$url .= '&filter=' . $this->request->get['filter'];
			}
			
			if (isset($this->request->get['sort'])) {
				$url .= '&sort=' . $this->request->get['sort'];
			}
			
			if (isset($this->request->get['order'])) {
				$url .= '&order=' . $this->request->get['order'];
			}
			
			
			
			foreach($limits as $value) {
				$data['limits'][] = array(
				'text'  => $value,
				'value' => $value,
				'href'  => HOST . str_replace_once('&', '?', $url. '&limit=' . $value)
				);
			}
			

			$url = '';
			
			if (isset($this->request->get['filter'])) {
				$url .= '&filter=' . $this->request->get['filter'];
			}
			
			if (isset($this->request->get['sort'])) {
				$url .= '&sort=' . $this->request->get['sort'];
			}
			
			if (isset($this->request->get['order'])) {
				$url .= '&order=' . $this->request->get['order'];
			}
			
			if (isset($this->request->get['limit'])) {
				$url .= '&limit=' . $this->request->get['limit'];
			}
			
			$url = str_replace_once('&', '?', $url);			
			

			$pagination = new Pagination();
			$pagination->total = $product_total;
			$pagination->page = $page;
			$pagination->limit = $limit;
			$pagination->url = HOST . ($url . '&page={page}');
			
			$data['pagination'] = $pagination->render();
			
			$data['results'] = sprintf($this->language->get('text_pagination'), ($product_total) ? (($page - 1) * $limit) + 1 : 0, ((($page - 1) * $limit) > ($product_total - $limit)) ? $product_total : ((($page - 1) * $limit) + $limit), $product_total, ceil($product_total / $limit));
			

			if ($page == 1) {
			    $this->document->addLink(HOST, 'canonical');
				} else {
				$this->document->addLink(HOST . ('/?page='. $page), 'canonical');
			}
			
			if ($page > 1) {
			    $this->document->addLink(HOST . ((($page - 2) ? '/?page='. ($page - 1) : '')), 'prev');
			}
			
			if ($limit && ceil($product_total / $limit) > $page) {
			    $this->document->addLink(HOST . ('/?page='. ($page + 1)), 'next');
			}
			
			$data['sort'] = $sort;
			$data['order'] = $order;
			$data['limit'] = $limit;
			
			$data['continue'] = $this->url->link('common/home');
			
			$data['column_left'] = $this->load->controller('common/column_left');
			$data['column_right'] = $this->load->controller('common/column_right');
			$data['content_top'] = $this->load->controller('common/content_top');
			$data['content_bottom'] = $this->load->controller('common/content_bottom');
			$data['footer'] = $this->load->controller('common/footer');
			$data['header'] = $this->load->controller('common/header');
			
			$this->response->setOutput($this->load->view('product/category', $data));
			
		}
	}

Смотря кому, и для чего.
Вы уже описали три вполне рабочих варианта, но два первых, это конечно, если для себя.
Третий имеет право на существование, уже и для заказчика, но, пожалуй, лучшим вариантом будет написание соответствующего расширения.

Больше и вариантов в принципе нет...

В:

install\cli_install.php
удалить

if (!function_exists('mcrypt_encrypt')) {
$error = 'Warning: mCrypt extension needs to be loaded for OpenCart to work!';
}
И в:

install\controller\install\step_2.php
удалить

$data['text_mcrypt'] = $this->language->get('text_mcrypt');

...

$data['mcrypt_encrypt'] = function_exists('mcrypt_encrypt');

...

if (!function_exists('mcrypt_encrypt')) {
$this->error['warning'] = $this->language->get('error_mcrypt');
}

Столкнулся со схожей проблемой. Решения в интернете не устроили. Сделал свой вариант, может кому пригодится.
Добавляем в catalog\model\catalog\product.php новую функцию

// получение пути шаблона
public function getProductLayoutRoute($layout_id) {
	$query = $this->db->query("SELECT * FROM " . DB_PREFIX . "layout_route WHERE layout_id = '" . (int)$layout_id . "'");
	if ($query->num_rows) {
		return $query->row['route'];
	} else {
		return "";
	}
}

Меняем в catalog\controller\product\product.php строку

$this->response->setOutput($this->load->view('product/product', $data));

на

$layout_id = $this->model_catalog_product->getProductLayoutId($product_id);
if($layout_id) {
	$route = $this->model_catalog_product->getProductLayoutRoute($layout_id);
	// проверяем существование файла шаблона
	$filepath = DIR_TEMPLATE . $this->config->get('config_theme') . '/template/' . $route . '.tpl';
	if(file_exists($filepath)) {
		$this->response->setOutput($this->load->view($route, $data));
	} else {
		$this->response->setOutput($this->load->view('product/product', $data));
	}
} else {
	$this->response->setOutput($this->load->view('product/product', $data));
}

Далее, создаем копию product.tpl и называем его, скажем, zakaz.tpl. Контроллер копировать НЕ надо.
В админке добавляем макет Zakaz, указываем путь к новому файлу шаблона product/zakaz (без tpl). Идем в товар, меняю ему дизайн с Product на Zakaz.

Скорее всего Вы используете php 7.2, в нем mcrypt убрали как устаревшую функцию, верните на 7.1, для OpenCart 2.3 версия php выше не нужна.