I understand your frustration. I've had some trouble too, but that wasn't really GM:s fault. I ordered my motor and battery from
www.conhismotor.com. As I've understand, it's the same motors that GM sell. Customer service wasn't great but everything got here in one piece so that's something. Communication with the Chinese can be a real pickle at times since they don't really speak proper english.
The best tip I can give you is try to find some local dealer:
http://goldenmotor.com/contact/contact.php It's always easier to communicate in a language both parties understand and it's easier to hold a local dealer liable if there are any problems.
(Also about your broken MP: what's the problem? Have you tried fixing it? It seems the internal controllers in the MP's are crap so you can always try an external one.)
Good luck!