Handle 404 query

/**
 * Handle 404's
 */
function prefix_event_posts( $result = false, $wp_query ) {
	if ( is_admin() ) {
		return false;
	}

	if ( 'your_post_type_name' !== $wp_query->query['post_type'] ) {
		return false;
	}
	
	$post_id = 20 // Post to load

	/* Load post */
	$post_to_load = get_post( $post_id );

	/* Create new post */
	$new_post = new WP_Post( (object) $post_to_load );

	$wp_query->post       = $new_post;
	$wp_query->posts[]    = $new_post;
	$wp_query->post_count = 1;

	return true;
}

add_filter( 'pre_handle_404', 'prefix_event_posts', 10, 2 );