We sell these freewheels on this page: http://www.electricscooterparts.com/freewheelclutches.html
Wow that gear motor shaft is definitely stripped out where the sprocket key seats into it. It must not have been properly hardened by the factory.
This kind of problem is very rare, in fact this is the first time I have ever heard of it happening, so we do not carry that gear in our store. Unfortunately the only solution that I would have is to replace the entire gear motor.
If you would like to attach a photo of the motor and let me know if it has any specifications printed on its label then I can see if we have or can get a replacement for it.