Be careful about amazon batteries particularly ones with fire in the name a lot of fakes.
On the cheap end i would do a single imr 18650 or 14500 if you want to fit a pair of them in the original battery compartment with some rewiring to put them in parallel or a li-po pack for a rc rated at 4.2v. The imr is good for 25-35 amps depending on the brand i would recommend efest or aw.
I would also look at PWM regulation since the motors are only rated for 2.4v going too much higher than that will burn them quicker the circuit is relatively simple and can be made small enough to fit in the gun or even the battery compartment depending what battery and where you put it.
If you dont want to put that much work into figuring out the power situation. 4 eneloops in the stock bay would power the motors with a slight over-volt would not provide the amps for stall though.
The main thing I am curious about is that the black things connected to the motors are not resistors they are inductors designed to drop voltage and increase current I don't pretend to know everything but would not having them in place help with current draw as the AA it is meant to run on would not provide the current to drive the stock motors well enough on their own.
For Ni-MH energizer claims 4c eneloops say 1c duracell claims 2c.
Edited by Lunas, 06 September 2014 - 07:49 PM.