Geish:

Your K&B Sportster 45 is plenty for the GP 1/6 Cub.

My wonder at your asking about the most powerful engine wasn't because of your newness to the hobby, but at the thought of spending high dollars for an engine you could well ruin in the process of learning to fly. And speaking of which, the Cub will be a good introduction, but not much of a trainer. Built well, it's just too easy to fly. Once it's off the ground, anyway. Take off and landing with a taildragger can be a challenge.

If you destroy the K&B get another inexpensive 46 engine, they are mostly in the same power range, differing mainly in engine life and price.

Bill.