Will this work?

add_filter('wpfts_index_post', function($index, $post) {
	global $wpdb;

	// Basic tokens
	/*
	 * This piece of code was commented out intentionally to display things
	 * which was already done before in the caller code
	$index['post_title'] = $post->post_title;
	$index['post_content'] = strip_tags($post->post_content);
	*/
	if ($post->post_type == 'product') {

		$prefix = $wpdb->prefix;
                //easy way to get all taxonomy even from other modules
		$sql = " 
			SELECT * FROM {$prefix}term_relationships AS tr
			LEFT JOIN {$prefix}term_taxonomy AS tt ON ( tt.term_taxonomy_id = tr.term_taxonomy_id )
			LEFT JOIN {$prefix}terms AS t ON ( t.term_id = tt.term_id )
			WHERE tr.object_id = {$post->ID}
		";
		$taxonomy = $wpdb->get_results($sql,ARRAY_A);
		foreach($taxonomy as $t){
			$p[$t['taxonomy']] =$t['name'];
		}
		$data = array();
		$data[] = get_post_meta($post->ID, '_sku', true);
                //for parts with diff SKU with diff vendor stored in custom fields
		$data[] = get_post_meta($post->ID, 'MPN Number', true);
		$data[] = get_post_meta($post->ID, 'MPN2 Number', true);
		$index['sku'] = implode(' ', $data);
		$index['product_taxonomy'] = implode(' ', $taxonomy);
	}
	return $index;
}, 3, 2);