well, I'm learning this as well.

What I have learned so far is not use a gasser below a .61 two stroke size. So I convert the .61 two stoke to an equivalent 4 stroke. (For example ... a .61 two stroke would approximate a Saito FA-100 four stroke.) Then to convert to a gas engine a basic conversion seems to be methanol four stroke fuel size engine multiplied by 16.3. That suggests that a .61 two stroke would use a 17 cc gas engine. A 120 would use a 20cc gasser. A 150 two stoke is equivalent to a 26 cc.

That is why many people are interested in learning more about the new Fox 15 cc gas engine. Another way to look at comparison though is weight differences. A gas engine probably is 15%-20% heavier than a methanol fuel engine. I'm not sure why. That's why I suppose the minimum gas engine (technologically speaking) so far seems to be a 15cc.

I've also learned that when anything is larger than a 1.50 cu four stroke equivalent, a gas engine is the best choice.

But I'm still perplexed by net power ... for example power-wise speaking, is a 26 cc gas engines as powerful as a 1.50 cu alcohol fuel engine?

I guess the other way to do a conversion is weight. A 9-12 pound model is a 26cc gas? A 12-16 a 30cc gas? etc. Still learning that.