The Pro 91 exists. I bought mine from Quantum Models. It's on a Model Tech Delta Fighter 90, with a Performance Specialists muffler and a APC 12x8 prop.
My engine, about 30 days old, had low compression out of the box and proper run-in didn't improve it. It runs well with excellant power but I'm about to send it back to Ace Hobby for repair. It has a ringed piston. Nice looking. OS91FX muffler also fits exactly. I understand this engine was produced to power the Rare Bear pylon plane. Selling price was $199.