A Cub with that weight and wingspan would do better with a .70 four stroke but not a lot. It would mostlikely be sufficient for 'mild' aerobatics like stall turns, rolls and the typical 'Cub loop' which usually turns out to be egg shaped rather than round. However, before you change engines, try entering your loop with a slight dive to build up speed prior to pulling up elevator to attempt a loop. This small amoung of 'energy management' should help you out.