What is it putting out right now? What do you wish to see increased?
Mp 40 may not fit a 9.6 battery so if you want to increase ROF you need a new motor or switch to Lipo (add a MARK III mosfett for safety).
For Fps use a m110 spring along with A bearing spring guide, V3 cylinder head, bearing piston head and polymer piston (1 metal tooth).
If you want a cheaper fps increase and willing to fix broken parts just drop a m120 spring and cross your fingers
Either way replace hop bucking and nub with scs spacer and hard rubber bucking.
That should do ya some good.