I put a 20cc stihl conversion on a 60 size four star. It flies fine but is a bit heavy. I think that lighter will fly better even with a bit less power. We'll see, I am currrently messing with an old Magnum .91 2-stroke. I got it running and will put in in the four star and see how it goes.

I did a Weed Wacker 25 cc featherlite a while back. Even with porting, open exhaust, bigger carb and electronic ignition, it was no power house. I chopped off everything I could and it is not very light given the power output.

It seems like smaller conversions work best on short nosed slow planes, Cubs and WW-1 war birds.

For a 40 size plane I woulld look for a ringed glow engine and install a rimfire plug and RCEXL ignition. Install the magnent on the hub and go. Run the ignition on AAA size NIMH batteries. The Super Tigers are very reasonable. The 51 and 61 ringed might be pretty good?

http://www.supertigre.com/engines/

Perhaps even some other 60-ish sized ringed engine that is at hand? If a tiny walbro carb show itself then go ahead and make an adapter. Otherwise it will run wih the stock carb. Just run lots of oil in the mix to protect the big end of the con rod. Yes it would be nice to have bearing on the con rod but it is not necessary. Hey, they always did it that way on the old ignition engines. Wasn't he most concearned about the price of fuel. This solution would work. Yep, the plane would get slimed but, that was not the issue for him.