Hi!
I think you should be able to do it. One file must be replaced, and that should be it. Please make sure to have a full site back-up just in case.
1. Open up the wp-content/plugins/ajax-search-pro/includes/classes/core/class-asp-manager.php file on your server
2. Change it’s contents to this.
3. Save the file.
4. Save the search instance options as well on the plugin back-end.
Hopefully this should do it.