And unless you are happy taking a V2 gearbox apart, leave it to someone who knows, as searching around for all the small bits when the box flys apart because you didn't hold it all together can be very frustrating.
You'll also need to know what spring is in there first. See the spring guide on the forum to gauge what you will need, on the basis that everything else is working correctly.
Poor fps can be down to broken hop rubber, poor piston head O ring seal, poor cylinder head seal or ill fitting air seal nozzle, not just a lower strength spring.