Richard, You may want to post in a different forum, scale. You may get a faster response, but I can give an answer. When that plane was designed the typical 60 engine had the same power than a New 46 to 55 engines. Any New 60 or 75 (2 stroke) should fit and have more than enough power. OS, Thunder Tiger, or Tower. All this depends on the total flying weight and how aerobatic you want it. Greatplanes has a Scale muffler and header that could help hide the muffler, mounts on the firewall. If you don not care about the head sticking out, an OS 91 4 stroke or 120 would work also and the muffler installation would be easer. Performance Specialties has a swivel muffler that works great. Good luck, nice flying plane.

The OS 120 4C would be my pick if the plane is 10 to 12 pounds.