Maybe try putting want add on here for a motor. Even a motor that needs an overhaul should be available.. I have found plenty on Craigslist but I also search every state for parts.. I think the Kawasaki kx500 is a dirtbike motor that has a counterbalanced. But I have been wrong before, also those r probably more rare to find than a ZILLA motor. Again I would just do some searching on Craigslist and find one. Have it shipped if need be. For the amount of work to get a banshee motor to fit and work you might as well just buy a banshee. They are everywhere and can find them for cheap.
The kx500 wasn't counter balanced the only 500's bikes that are counter balanced are a lt500 or a trx250r with cr500 top end or a saber top end and one conversion kfx250 tecate 4 bottom end with a kx500 top end.