Just a few points to consider. First and formost... CLIP THE WINGS! A clipped wing Cub will still float like a dream, but it will be capable of some really cool aerobatics also. The full size wing is like trying to get a surf-board to fly (if you're not looking for scale flight)

Something else to consider in the engine dept. is an OS 70 Surpass. This engine is PERFECT for a 40 size Cub!!!!