The quickest way solve the problem: Slap on a bigger engine. With enough power, just about any airplane will do an inverted loop. Having plenty of altitude can't hurt either.

"Remember, if you're in trouble, hang on to your altitude. No one has ever collided with the sky."

-Dan Nelson

PS: The way to tell if you have too much power? If you give it full throttle and the engine mount rips out of the plane.