Is it a fixed slide or Blow Back pistol?
If blow back IIRC they should be firing at something like 260 (on green gas?) if CO2 it's a bit higher but I think should still be under 350 FPS. If it's CO2 and fixed slide you're kind of up the river, it's cheaper to buy a new gun.
If you can, try and chrono it first before going through all this work.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE