Good to hear you are looking for a killer play bike. The banshee will do it for ya for sure. It's definitely not the best for MX track carving, but you can make that thing really shine ice racing!
As far as a motor like that, I hate to break it to you, but you need to go all the way through it. I would plan on spending quite a bit of cash to get it right the first time and be done with it. If you were just hoping to put a quick top end on it and having a solid bike you need to part it out and quit while you're ahead.
I'd suggest that you do a fresh welded crank, but since you're already buying a crank, just get the 4mm stroker crank for not much more coin. The. You need porting and domes to work with the new crank. Then there is the supporting mods that go with your type of riding.
Pipes, reeds, carbs, manifold, head, timing plate, any new hardware or fasteners, pancake bearing, shifting mods, fresh clutch, etc. it can get expensive pretty fast. So, like I said. If you wanted a fast cheap project, just ditch it and buy something else more complete. But if you are ok with spending the coin, stop wrenching, and do a ton of research on here and ask questions you can't find answers to.