I flew one for two seasons and sold it in perfect shape. It had an OS 55 with a Macs pipe on it. I just flew the heck out of it and finally got bored so I sold it while it was in great shape. That being said I would suggest the same engine or the newer OS 95 four stroke engine which is awesome. I have it installed in another plane, Fokker D7, but it hauls this heavy bipe around with no problems. Its a very light compact engine with excellent performance.
You need to give this plane a little extra power to keep it fun and edgy. I would say the 55 was just OK and good average match for this airplane. If I did it again Id use the 95 or a two stroke 60 high performance engine.
Another point would be a small mod to the wing to build inboard flaps to help with landings. I was close to doing this before I sold mine. Now I want the Revolver 70 someday but Id do the same on that one!