i read some article about one guy in the coubtry who "resizes" piston sleevs crushes them or something.
if you really had to do it i think researching the O.D. of your current piston sleeve and try to match it to another piston and sleeve from a larger displacement engine. i might look into this just out of curiousity i think you would probably end up boring the engine casing to fit and you would definetley have to take into consideration connecting rod travel as wel as stroke.