If this uses the same gearbox/motor combo as the J8472, it's a 17T motor with a 15-17T 7R gearbox.
If it fits (I don't know what the space around the motor looks like), the cheapest
way to get more torque is to order two Nichibo 7013F "7" series motors ($16 for 4, shipped to your door), two 16T pinions (5mm hole, 32DP), and either fabricate two spacer/centering sleeves (1/2" PVC pipe and an 11/16" drill bit) or buy them from mltoys (775 to 7R mounting kit - $11).
On 12V this will give you about 40% more torque and it will be slightly slower (16T vs. 17T means 6% slower). Unlike other 775 motors (like the 8514/Banebots 18V), this won't stress your wiring & battery setup.
Example pinion gear:http://www.ebay.com/itm/HobbyStar-32DP-Pinion-Gear-16T-Hardened-Steel-5-0mm-Bore-5mm-32-Pitch-16-32P-/171955600929?epid=525168309&hash=item28095a3e21:g:-0cAAOSw8GhZhNDf
Mltoys mounting kit:http://www.mltoys.com/product-p/775-to-7r-mounting-kit.htm
Note that these motors come without any flats on the drive shaft end and the pinion gears use a set screw, so you need to file/grind a flat into the shaft.
Or you can run what you have at 18V - -this will increase the torque of your current setup by roughly 50% as well as the speed -- but you'll probably burn through motors fairly quickly if you're doing a lot of hill work.
The 775 Nichibo motor I linked should take 18V without any issue.
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (NOT all-wheel-drive), and fading color
Hurricane - 24V w/50A step-down controller, Mabuchi 8514 motors
Princess Mustang - 12V w/23000 RPM 550 motor and 23T gears