searchwp-customizations.php

<?php

// Allow searching by WP_Post ID.
function swp14237_searchwp_results( $results, $attributes ) {
	$search_query = get_search_query();

	if ( ! is_numeric( $search_query ) ) {
		return $results;
	}

	$the_post = get_post( absint( $search_query ) );
	if ( is_null( $the_post ) ) {
		return $results;
	}

	array_unshift( $results, $the_post );

	return $results;
}

add_filter( 'searchwp\query\results', 'swp14237_searchwp_results', 10, 2 );