Can I mount my os 70 s ll sideways on my Cub.

that is the preferred mounting method on a Cub. The head sticks out the side of the cowling, like on a real one

Absotutely!
Dennis-

Another + is that you'll hide the muffler under the plane so it'll be less visible than having it protrude out to the right of the cowl as it would with a 'normal' upright engine mount.