It could be a 25cc or a 30cc. I have converted a few of these, and they run great. They are a little on the heavy side and are not super powerful, but will fly 12-14 lb. planes. You'll have to remove most of the aluminum flange that's around the front of the crank case, but leave enough to hold the coil in place. I did this with a hack saw and files. You'll also want to remove the fins from the front of the flywheel, and the steel pins and tabs and springs that are on the front of the flywheel for the recoil starter. Most people have the front of the flywheel machined flat, but I cleaned it up with files too. It has to be as flat as possible. There might also be a little hole in the cylinder above the exhaust opening which should be plugged to increase compression. I don't remember the backplate being plastic, but metal.

I got my conversion kits from carrprecision.com. It consists of a turned prop hub, which slides over the crankshaft, and a mounting plate which attaches to the back of the engine, on top of the back plate. Check out Carr's website for more info. and look at their model gallery, too.