Got the same car for my son. Probably my favorite RO based on looks.
First thing I did was put a 7.0 Ah 12V battery in it. IF you unscrew the blue casing around the factory battery you can just cut the leads off and put female disconnects on it for the bigger battery. 12V battery fits in the factory slot with the velcro strap perfectly.
After a week of use one of the teeth of the gearbox broke. Couldn't tell if it was takeoff or braking. Anyways I called Ballard Pacific (manufacturer) and they sent me a new gearbox with the motor attached and also a coupler (washer adapter that goes between gearbox and wheel). They sent it free because it was under warranty, but if you bought it from them it's $25. In the mean time I 3d printed a replacement gear for the other one.
Great thing about this car is you already have the hole for the motor on the other side and the other wheel has the "kidney" shapes built in to interface with the gear box.
Problem is because there isn't a gearbox on the driver's side, the axle has a spacer welded to the axle to offset the wheel. It also has a set screw going through the body from the bottom into it to keep the axle in place. It is welding using two rosette welds. I drill these two welds out, cut down this spacer in half, drill a new screw hold and had it hold in place using hose clamps. Washer at end of spacer has to be ground down a little to allow it to go into the hole of the gearbox. You will see all of this when you look at it. Also you don't need the plastic cage that screws around the motor, they stay in place fine without them.
I pulled the pedal out and added a 1 ohm 5w resistor to the shorting side to damping the braking. Much better.
The motors included are 540 motors (should only use 6V) so I bought heatsinks for them off Amazon. Once these burn up, I will switch to proper 550 HPI motors (don't want to worry about dealing with reverse and forward tuned motors). 540 and 550 are the same diameter and mounting, 550 is just a little longer.
The wiring on this car is tiny and I will eventually rewire with 14 gauge.
If you don't do dual wheel you will be buying wheels quickly. My son drove the one-legged 12v setup for maybe a month and the "grip teeth" on the center of the wheels has already ground down from spinning on takeoff and braking. With the posi-traction dual motor setup at 12v there is no slip on take off. https://www.youtube.com/watch?v=6uaNmvkRR2Y&feature=youtu.be