You won't fly it at full power? so you'll either not push the throttle up so far, or will adjust the throttle not to go to full power.....then why in the world put a 15CC's weight in the airplane?
This is just way too much engine for that aircraft......and no a 15CC is not the same as a 60 2 stroke....the math says that it;s the same as a 90 2 stroke.
The other thing to consider is the 15CC will require a bigger prop. unless you put a longer strut on the nose or bigger tires you might not have the ground clearence.