if the gp cap is what your looking for the o.s. 1.6 is plenty of power. and is a lot cheaper. if your looking for wayy overkill put a 50cc in it. I would lean more towards a 40cc class engine. I have this plane with the o.s. other than the 2 stroke exaust notes it's a great engine. plane comes out right at 12lbs. for me a gas engine is a lot easier to 3d with than the 2 stroke but I'm getting use to it. here is a video of al lewis's cap flown by kyle woyshnis with the o.s. in it.
http://media.putfile.com/Kyle-3Ding-My-CAP-232