the star low-caps should work fine. no need to replace the gears. drop it down to a Systema M100 spring and put in a cylinder enhancement kit. this will improve durability and airseal, which will probably put you close to the 400fps mark. If you use the 120 spring, you will be over the FPS limit especially if you replace the internals.
http://www.airsoftcanada.com/SpringChart.html