Any AEG should work on a 9.6v mini NiMH battery, anything around 1500mah.
If it doesn't, then there's something wrong with your setup, weak motor or wrong gear ratio.
MOSFETs are nifty little nickel sized electronic boards that bypass your analog trigger switch. If you're running a 9.6v battery, or any LiPo, it's highly recommended you install one. Not only does it prevent your trigger switch from corroding or melting, but it also reduces the electrical resistance in your gun, raising your RoF a little and extending your battery life.
If you're not comfortable swapping out a motor, or diagnosing this stuff on your own, you can try finding a local gundoc to help you out!
Also give this a read if you haven't already, it's pretty well everything you need to know about MOSFETs and LiPo's