If it was me and money was not a factor I would assemble the plane and add weight where the engine mounts until it is balanced, then I would buy the 4 stroke engine that comes closest to balancing the plane.
Prop clearance would also be considered, I personally like a 13 inch prop on a 70 and a 14 inch on a 91 4 stroke engine.
Based on the published weight of the plane a 70 4 stroke will give you spirited performance that should allow giant round loops but not unlimited vertical.
If money is a factor go with the 2 stroke engine.