to know for sure 100% you would have to do a compression test but normally at sea level you can run 20-22cc domes on get away with premium...
no im not saying anythign about a big bore kit... drop big bore kit from your head... theres ways you can get some huge power out of your stock cylinders... get a good port job... that has nothing to do with your bore size. its your transfers intakes and exhaust ports....
get a head with the right domes..pistons ,crank, reeds, intake, carbs, pipes, cdi, coil....
you dont have to go big bore to get some major gains