If the airplane is of generous proportions, the ST G-,51 , hands down. i would even put one in a SIG Chipmunk if that group of engines was hat I had to choose from. If the model is down sized a bit from there, I would probably choose the LA.46. I like the classic run of the  ST.46, and if one had a good ring fit on a standard version, that engine can give what most people want. Put in a Brian Gardner ABC piston/liner conversion, and you get the benefits of what the LA.46 offers. I have flown a 60 ounce Legacy 40 with an LA.46 for power for several years. At it's size and that weight, I think that is the upper range for the LA.46. A ST.51 will have that extra grunt if you need it and if/when I build another, that's what it will have in it. The Legacy 40 is at 565 sq.in. for wing area, so that I think would be the limit for a LA.46 and anything close to 60 ounces. I don't know if even a converted ST46 would handle that. Having said that, a SIG Chipmunk is reported at 570 sq.in. and the ST.46 is what a lot of guys dropped into the nose of that airplane for best power. Anything at or over 600 squares, the ST.51 is the best choice I think.
   Type at you later,
    Dan McEntee

The ST G.51 is one of my favorite engines. So far, I haven't used the Brian Gardner ABC conversions, but have flown the same G.51 with a GMA-spec retimed R/C cylinder and also a stock CL timed cylinder. The engine seems to work very well with a wide variety of propellers...11-6 APC, 12.25 x 3.75 APC, 12x5 APC and 12.5 x 5 Thunder Tiger Cyclone. Easily flies a 65 oz 700-ish square inch ".60 sized" stunter.

I'm not a fan of the ringed piston configuration, but I also haven't had a need to change rings frequently or anything like that. It makes re-assembly a PITA due to the ring being pinned. The windows in the cylinder are pretty large, thus the pinned ring is required. I think the ABC setup will be awesome!  Steve