4mm serval or 4mm stock cylinder.
stock cylinder is your cheapest option as you already have the cylinders. so thats a 800$ (if ya dont get serval cylinders) saving already, (800 could be used for a 4mm crank and porting)
shearers wil be fine, cpi would be better. i trail my shearers and have good time (not tight trails tho).
cubs do wear the intake side of the piston, quite a bit, but people have ran em for years without a problem. tho as SR11 said, stock cylinder can be bored/sleeved ( ya need a decent problem to mess up the nikasil tho)
pro mod is a must, big difference from a stocker and ya can beat the hell outa it.
id go a 4mm serval tho, then its done and if you want more power you can port the serval cylinders.