The junk internals on the clone finally broke, not unexpected.
Luckily you can buy a whole new gearbox, ready to go:
http://www.airsoftparts.ca/store2/in...oducts_id=1158
Otherwise, trying to find the parts piecemeal... that's gonna be hard. Specially the shell, there's no aftermarket shell. So that complete Marui one is your best bet.
PS. you should make your friend pay for repairs. And don't let him mess with your guns again. Everyone knows firing an electric gun nonstop is a great way to break it.