Moki 1.8, I used to have that plane (Gene Soucy Extra) and it flew quite well on an OS 1.60. I know for a fact that any of the props used on a 50cc engine will be WAY too big unless you get taller landing gear for it (a 20" prop will work fine, but will require 3-point landings).

I was going to comment on the CRRC engines based on their past history, but the newer ones seem to be pretty solid. I have a good friend who just put together a plane with the kit-built 40cc engine and it is a very solid engine, runs great, etc. I think that is the one VVRC is selling for $279, but Jody assembles it and tests it before shipping. (He is an ASE-certified mechanic with over 30 years of experience, so he knows what he is doing). He will guarantee the engine will run.

That engine with a 20x8 or 18x10 prop would be just about perfect for the Soucy Extra. Figure the power will be somewhere in between a Moki 1.8 and a Moki 2.10, for a similar overall weight (gas planes carry less fuel than nitro planes, so overall weight would be similar).

This plane should fly very well. If you want to fly around, do lots of rolls/loops/spins, etc, it will be perfect. It rolls very well. I LOVED the color scheme too, it is highly visible and looks very sharp in the air. I had issues keeping my OS 1.60 running so most of my flights landed in deadsticks so I can tell you it is very well behaved on landing approach, but it seems to land faster than larger 50cc planes.

Note that if you want to do 3D type maneuvers you will have a tough time. The plane will hover, torque roll, harrier, etc,. but you have to be very quick on the sticks to stay ahead of it. It is harder to fly and a larger 50cc plane, and significantly harder than a 100cc or 150cc plane (which feel like they are on autopilot by comparison).