Chuck beat me to it but there is a lot of the info you need in the gas engine forum. When the plane calls for a 1.60 or 1.80 Ipull out a 2.4 40cc, a 50cc could often be better but a lot depends on the engine itself. Iget about the same performance with a Brison 40cc as the guys are getting with the DA 50cc but both engines are on the high end of price. There are several good gas engines being brought over from China for next to nothing compared to glow engines. Istill like the Zenoah engines with the mag ignitions but these new china dolls come with electronic ignitions and good mufflers so they are worth looking at.
Yesterday Iwas flying with a friend that has beenusing these{ {don't quote me on the make please, I can't keep them all in my head!!}} CCRP? When you see the make you will know it. It is a Chineese 26cc that sells for $180.00 with an electronic ignition and the muffler. Another friend just bought there 40cc and it didn't cost much more. Very light weight and a real little power house. Can't beat it for the money. You will hear a lot about them at the gas forum.
There are hundreds of threads about gas engine plane set up too.