You can go for high compression pistons in the stock bore . 12.5 to 1 will certainly perk things up . or BEET racing has a 265 kit. You just need to bore the stock cylinder out to fit. Should not cost much for machine work .Of course a free flow exhaust and filter with jetting and tuning will be required to get the most out of either setup
http://www.beet.co.jp/english/kawasa...50r/index.html