ORIGINAL: jester_s1
It's not as simple as just getting the displacement the same. Glow engines make more power for their displacement, so a gasser needs to be a little bigger to be a true equivalent. But then gassers typically make their power at a lower RPM which changes the way the plane responds to the throttle and what prop it needs. The change in prop brings a change in trimming, which may or may not make the plane fly better.
That's very true but it also applies when speaking of any different type of engine. It would be easier if it just stopped there. Brand and induction type also mater, if you put one brand next to another of the same size and prop there can be a huge difference. Then you have to factor in engine cost against the added power and quality/ factory/dealer support. Like most everything in this hobby, there is a big learning curve.