Flying mine with a Saito .60twin, which is equivalant to about a .46 two cycle I guess and it flies just great, nice and slow and scale-like. I am assembling another with a Saito .90twin for floats and should be more than enough power. I think your YZ 91 is going to be more than a Cub needs, but just manage the throttle and you'll be fine. Just remember, that's a big wing when you go into those high g turns!!

Russ