No Yamaha chainwheel fits. No nothing aftermarket fits pretty much.
The 41t for the Ducati Paso does fit the bolt pattern but is for a 530 chain, so the chainwheel needs to be turned down on the outside to fit the 520 chain. Does work and I have uses an AFAM chainwheel for the Paso myself.
You'll just have to order the replacement from MZ if you really want the steel cog again. I certainly would not!
Best place to go is here:
http://www.ls-motorradteile.de/
you can write your email in English:
sieg@ls-motorradteile.de
Leonard will CNC mill any chainwheel you want in short order. Just tell him it's for the Skorpion (he has a Skorpion himself) and how many teeth it should have. He will gladly also supply front sprokets and chains.
In a pinch, I can oder it from him and send it, thus presumably avoiding customs and the payment hassle of currency exchange. If that is an option, write my offlist.
There must be a company in the states milling chainwheels, tho.