[Azathoth]

You are best served with a full tune up kit like the one linked below. Pick the brand that best serves your budget. Keeping in my that each brand has it's own sets of flaws. Their are MANY FTK's (full tune up kits)

http://www.redwolfairsoft.com/redwol...Expert_Set.htm

Guarder
Modify
G&P
Systema
Prometheus
Jing Gong rama lama ding dong
and many more

Given the brands above I would advise you go with Prometheus or Modify. If it was my gun, I would custom build from the ground up, probably use a Prometheus FTK to replace the old TM parts and sell the gears and get siegeteks.

The stock TM gears will not last very long with a M120 spring, the bushings will not be able to handle the stress of the additional load from the spring and gears and if those fail the gears fail, as possible the tappet plate.

I use Prometheus Hard pistons in my guns or the System Supercore. Modify makes decent pistons at all the price ranges as do some other brands. You will want a V2 Piston, with a half rack of metal teeth. You will want a set of bearings on the piston head.

The Battery compartment for the P90 is very small. I recommend LiPO power 11.1v. AEG's that are shooting ~400fps In my opinion should have a MOSFET and be running SubC sized cells for best performance not AA or 2/3a (again my opinion but almost everyone runs 2/3a in some cases it's required due to space constraints but that excuse went out the door when LiPO/LiFePO became cheap). You realize that the J type battery requires you to mod your back plate cover and the plate that screws down the gearbox?

If you are going to Lithium power I do recommend replacing the wiring with some quality 12-14awg milspec wire. You will want to anyways if you are installing a MOSFET.

Motor, It's fine, but you don't NEED that motor. The stock TM motor will do the job as will some other brands such as Tienly, G&P 160 etc. You won't see anything near near the maximum power or potential of that motor with the battery you have selected. The Magnum doesn't get to show off whats under the hood until you start pulling M150++ and/or with 10.8v++ power sources. If you are going Magnum motor I highly recommend you do the insulation mod.

Go with the type 0 cylinder. You have extended the barrel length of the gun and the cylinder that you selected will not have the volume of air available for you to get the velocities that you are asking.

Spring:
The spring chart is a bit out of date. It's decent guideline but it's certainly dated information.

If you go with just the parts that you originally posted your gun will cycle. But it may not last more than 10 shots or it may last 1000 or so. The TM gears are certainly not up to the task of M120, and the bushings should be replaced ASAP.