Either one of those kits would work well, but you'll need carbs if you want 80 + Hp. At least 34mm.
As for the crank, yes a 4mill does make a huge difference, like night & day. My bud has a stock stroke cheetah and I have a 4 mill. I run rings around him. If you buy a engine kit you should buy a new crank anyway so go with the 4 mill.