Filter SearchWP results by post type.

filter_search_wp.php

<?php
function filter_search_by_post_type( $included, $engine, $terms ) {
    $post_type = get_query_var('post_type');

    if (!is_array($post_type)) {
        return $included;
    }

    $posts = new \WP_Query([
        'no_found_rows' => true,
        'update_post_meta_cache' => false,
        'update_post_term_cache' => false,
        'post_type' => $post_type,
        'fields' => 'ids',
        'posts_per_page' => 999
    ]);

    $ids = $posts->posts;

    return empty( $ids ) ? array( 0 ) : $ids;
}
add_filter( 'searchwp_include', __NAMESPACE__ . '\filter_search_by_post_type', 10, 3 );