500 miles is good for piston ring changes! The TP due+ is a nice cylinder to go for - cast iron and includes a single ring allowing less frictional properties. There are many good cylinders out there: Mhr rep, due+, sp2, s6r - It all depends on how you want the bike - Speed over reliability?
What other gear is going to be used, ie exhaust, carb, inlet, ign etc..
Regards
Butch