You need to remove the pull start. Imagine that thing kicking back and pulling your hand right into the prop. Plus, what if it starts, and you let go of the rope, then the rope ends up lashing into the prop disc...just dangerous.
1/4 scale is just right for that sized engine. Maybe a 90 to 100 inch wing????
Also, I wouldn't use a 3 bladed prop on a cub.....it would just not look right.
you need to machine a lot of the extra weight off of the engine. Shave the flywheel down-take off the cooling fins. Also, shave down some of the cooling fins on the head itself. Get a free-flowing header, or make one from aluminum stock. You'll have to have a prop driver machined for the engine as well.
All in all, I would think the afore mentioned 150 dollar 25 CC with EI is a better deal, unless you are just wanting to tinker with this thing. I would guess you'll have at least 100 bucks tied up in the conversion in both cash and time.