Assign a template to a single custom post type page

cpt-template.php

<?php 

// set custom post type templates in post meta
add_action('publish_name_cpt', 'prefix_add_template', 10, 2 );
function prefix_add_template( $post_id, $post ) {

  if ( $post_id == 1234 ) {
    add_post_meta( $post_id, '_wp_page_template', 'your-template.php', true );
  }

}

// use template in front end
add_filter( 'single_template','prefix_get_template', 10, 1 );
function prefix_get_template( $template ) {

  global $wp_query;
  $post = $wp_query->get_queried_object();

  if ( $post->ID == 1234 ) { 

    $post_template = get_post_meta( $post->ID, '_wp_page_template', true );

    if ( !empty ( $post_template ) && $post_template != 'default' ) {

      $template = get_stylesheet_directory() . "/" . $post_template;

    }

  }

  return $template;
}