Get product link

WooCommerce default customer-complete-order.php email template does not link the order item to corresponding product. You need to use woocommerce_order_item_name filter and update the item name to have link. I just tried below code and it adds the link to order item (product name). Put this code in functions.php file of your theme. Hope this helps:

add_filter('woocommerce_order_item_name', 'woo_order_item_with_link', 10, 3);
function woo_order_item_with_link( $item_name, $item, $bool ) {
    $url = get_permalink( $item['product_id'] ) ;
    return '<a href="'. $url .'">'. $item_name .'</a>'; 
}