just curious to know what would be the choice engine to power a 4.5lb to 5lb highly aerobatic plane.
I was thinking a YS63 4 stroke or a 70 4 stroke?

Maybe a Fox .46 would be best? Lots of engines that can fill the bill, its really up to your personal preferance.

If you want a 4 stroke I think both of those engines are fine. But its really hard to generalize on just the weight of the plane. I have a 4.5lb trainer that flies on a 36 2 stroke and then a 4.5lb 3D plane that flies great with a magnum 90 4 stroke. There is a huge difference between the power of those 2 engines but both fit the bill for the plane.