Pistons commonly wear out on our bikes and things get slappy when the mileage gets high. If your bore is still well within service specs for out of round (coke bottle wear effect) and
your piston is not scored consider rematching your existing parts to the correct running tolerance using a piston coating on the piston itself.
Bumping the piston up to a correct running tolerance to the worn bore is much less expensive than reboring the cylinder and buying an oversized piston. You can save nearly $100 by recovering your existing parts by coating the piston.
There are many shops offering this service (hit your browser with "piston coatings") so shop around to find the best deal closest to you.
Swain has been in the business since it started and their low friction coatings seem to be able to bridge more gap than some of the others can do.
http://www.swaintech.com/store.asp?pid=10964Note: if your piston comes back a little bit tight to the bore or has a high spot on it, fine 600-800 grit wet or dry very carefully applied to the tight spot can make up a proper fit again.
It can be sanded gently where needed, it is just a very tough slippery baked on paint job after all ....