It depends upon the hp of the gas engine your are considering and your type of flying ... for me, in general, I multiply the four-stroke value times 16.4. That will get you in the ball park anyway IMHO.

So a 1.20 four-stroke would be about 20cc gas. A 1.4 four-stroke would be about 26cc. A 1.6 four-stroke maybe a 30cc.

But consider the aircraft weight, too. You can get away with a smaller fuel tank which might equal out the weight increase due to the ignition and battery. Sometime you get lucky ... DLE-20 ends up being about 2 ounces lighter than a Saito 1.50 and produces about the same hp.

So there is no pat answer.

My 2 cents.