Are you doing all work yourself? Do realize you won't be able to race in 250 classes any more? Well, I guess if no one protests. One guy snuck in a 600 into 250 class last year at Sears Point. That wasn't too obvious! Some related threads:
https://www.ninjette.org/forums/showthread.php?t=187321
https://www.ninjette.org/forums/showthread.php?t=257420
https://www.ninjette.org/forums/showthread.php?t=263401
Personally I would sleeve it and use ZX6 pistons... Not sure what to do about shorter pin-height on 300 piston. Maybe make it up with thicker base-gasket.