You're getting a lot of good advice here.

Weight: Take a look at a glow engine. Now take a look at a weedeater engine. The whole design is different. The gas engine has a flywheel, magneto, spark plug, diaphragm pump carb, and some other things that quickly add weight.

Power: Hard to compare there...for example, my last 1/4 Fokker Dr.1 used a 23 cc gas engine. The recommended engine was a 1.20 4 stroke glow. I wouldn't want to have flown it with the glow engine. The prop that engine could have turned would have been too small to give me the reliable flying characteristics I wanted.

Model weight: Not that easy. It depends on what flying characteristics you want from your plane. A giant scale Corsair might use a Zenoah G-62, let's say, but a Cub the same size might only need a G-38.

Back to power for a minute. You're looking at two stroke glow, four stroke glow, and two stroke gas. They all have different power ranges and prop capabilities. In picking an engine, decide how you want your plane to fly, factor in the weight, wing loading and drag, and choose an engine/prop combo that will do what you want it to. On my Dr.1, I wanted to swing a large diameter/low pitch prop. This would give me plenty of torque (in the large diameter), and limit the top speed (in the low pitch). For cleanliness, reliability, and sound, I wanted to use a gas engine. I looked at the gas engines available that fit the model's needs and chose the Zenoah G-23. Anyone who ever saw it fly said it looked great in the air and flew in a very scale-like manner. I'm not pushing the Zenoah, just citing an engine and model I have extensive experience with.

Keep asking, we'll keep helping.