If Specific Product by id sold then custom message on thank you page code

Use the hooks available on that end-point to create conditional messages. If you’re unsure which hooks are fired on that end-point, have a look at the excellent https://hookr.io/plugin plugin.

Suggest not changing the page template if it can be avoided. Try something like this in functions.php for your child theme:

<?php
  // woocommerce_thankyou hook
  // code goes in functions.php for child theme
  add_action ('woocommerce_thankyou', 'myfunction');
  function myfunction($order_id) {
    $order = new WC_Order($order_id);
    $items = $order->get_items();
    foreach ($items as $item) {
      $item_metas = $item['item_meta'];
      $product_ids = $item_metas['_product_id'];
      foreach ($product_ids as $product_id) {
        if ($product_id == 1) {
          echo '<p>You are lucky!</p>';
          break;
        }      
      }
    }
  }