Good luck finding a spare in the U.S.
These bikes got stipped of their motors and scrapped in favor of a Ron Woods frame and dirt tracking.
The 500cc head is ecspecially desiarable due to it being a smaller chamber than the later 600cc variation.
You might be better off trying to do a motor swap with a 600cc honda or ktm.
These motors were not as nearly as popular.
I would put a lc4 motor in it anbd call it even.
If you are into a full restoration you are going to have to pay the piper.
Rotax motors are not being given away.
I would get a cycle news and find a team with a spare motor.
Btw, these things have been USED!
Junk rotaxes abound.
Buying a motor that you haven't heard actually run is a serious risk.