Redirect 404 errors to posts with old path stored in custom field, if it exists

functions.php

<?php

add_filter( 'redirect_canonical', 'my_404_no_guessing', 10, 2 );

function my_404_no_guessing( $redirect_url, $request_url ) {
    if ( is_404() ) {
		// Drupal redirects
		$posts = get_posts( array( 
				'meta_key'   => 'drupal_path',   // change to your meta_key
				'meta_value' => parse_url( $request_url, PHP_URL_PATH ), 
				'post_type'  => 'any',
				'posts_per_page' => 1,
				'fields' => 'ids',
			) );

		if ( $posts ) {
			$redirect_url = get_permalink( array_pop( $posts ) );
		}
		else
        	return false;
    }
    return $redirect_url;
}