I would have a nice port job done to the cylinders while you have it appart. After pipes, a port job is the next step up in power. A head will add midrange but wont be as noticable as a port job.
When you port the cylinders they opean up the ports wich change the durations. It is like adding a cam shaft to a 4 stroke.
New prolite pistons
Port job from HJR (cheap and quick turn around)
A Noss Machine head if you have cash left over from the port job. If not a shaved stock head will do.
http://www.herrjugsracing.com/Porting_Motor_packages.html
If you just send the jugs to kevin he will bore them to the correct size and will have the pistons on hand and probably cheaper than most places sell them for.
I would get the Hot Trail port or the Dune Port job if it was me.
josh