pistons are designed to wear out before the bore but if the rings are not serviced regularily the piston skirt can wear a "slap groove" at the lower end of the bore and the rings can wear a lip into the very top of the bore. once this wears out, a new same size slug (piston) will have too much clearance and will wear out prematurely as well as being down on power, requiring the bore to be opened up (rebore) to take the next oversize slug so the engine is all back in spec. (correct tolerances). if there has been a catastrophic event...airfilter fail or the worn rings/ piston fail and gouge the bore, it may take two or more oversize machinings to clean the gouges out of the bore. if the bore is too badly damaged, the liner can be pressed out and a new liner pressed in.
so take the piston and the cylinder to the machine shop and get a measure done. determine (don't purchase) what oversize pistons are available. get the shop to bore to the next oversize and check the bore is now serviceable. if not they will needto machine it out to the next "next" oversize then you will know which piston to purchase. once the piston arrives, the shop will then hone (very fine rebore) out the cyl to give the correct piston clearance and finish the bore with crosshatch marks for better lubrication.
a tip.. the piston ring grooves have pins that locate the end of the rings...make sure you get that all lined up before slipping it back into the cylinder. its to stop the ring ends moving around to the exhaust or inlet port and then being grabbed and torn out...very exciting I did it once