RE: .30 or smaller 4 stroke engine?
What TFF said. I owned an OS 20 4 stroke for a while & while it was cute & a great runner it wasn't very powerfull, about the same as a sport 15 2 stroke.
My way of thinking is for a scale model fit the largest 4 stroke that can fit in the cowl. The best noseweight is aluminium & you can always throttle back. - John.