Being that it's a '91 model, I would go 0.025 over on the cylinder, piston, and rings, because it probably has a lot of wear in the cylinder. Boring a cylinder isn't really going to give you any noticeable power increase. That just cleans up any scratches or wear in the cylinder walls.
Personally, I would stay away from any high compression pistons for the utility ATVs, because the closer to stock that you keep them, the more dependable they are. If you're not worried about dependability and are after more performance, then you might consider it. But without other mods, I don't know if it would give you much more in performance or not.
I believe the low compression ratio is what causes those engines to last so long.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.