Any 50cc will do, it all depends on the price you want to pay for the engine and what kind of performance you're looking for out of the combo. I had a US41 in my Pitts for a while and it flew fine but I had to fly the plane not the engine and I had to be ahead of the plane to keep it out of trouble. The engine had been in a crash and fixed but had an unreliable idle so I put a Fuji 50 in the Pitts and it flies great. It doesn't have unlimited vertical but it has plenty for my flying and there is enough power to get out of trouble. Sometimes it hand starts with no trouble and other times it likes to have a starter put to the spinner.

Steve