I'm not so sure on that one. It definitely isn't a direct displacement thing going from glow 2-stroke to gas 2-stroke and it may not be going from glow 4-stroke to gas 2-stroke either. You have to consider an alcahol/nitro burning 4-stroke vs. a gasoline powered 2-stroke may have different power output given a similar displacement. I could be wrong too of course.

I'd lean more towards a 40cc-45cc but that's just what comes to mind first. The type of airplane makes a big difference too. For instance, a big Cub can fly on a lot smaller engine than a smaller aerobatic plane.

6inarow, what is the size and weight of the plane? What kind of planes is it (i.e. sport, scale, aerobatic, etc.)?