I’m not sure, but I think this will work.
$post_id = intval(get_option('your-option-name'));
if (!empty($post_id) {
// set global $post to this page
$post = get_post($post_id);
setup_postdata($post);
get_template_part('page'); // get the page teamplate instead of using this one
exit; // important
}
// if the post id is empty all of the standard/default 404 page below
