For a motor. It depends on what your looking for. But I usually have good luck going with a TM EG1000 for a torque motor or a hi speed. I wouldn't add one if you didn't change the internals. Unless you added a LiPo or tighter spring.
The one advantage about THIS gun in particular is the bushings are metal and it is factory shimmed meaning that tedious work is all ready done for you
But I wouldn't add a LiPo unless I had a piston with full metal teeth, a AoE, and the motor.
Ah, the anti reversal latch. You really do need this. It prevents your piston teeth from stripping and expands the life of your gun. I reinstall it by putting it in before the bevel gear, then when I put the bevel gear in, I push it to the left horizontally (not pulling it out) then slowly release it. A lot like setting a mousetrap. It will easily screw up when putting the shell on so just have patience and don't freak out. (It sticks better if you put some white lithium grease in the hole on the shell that holds the latch) mechbox.com if you need any video.