How to Supercharge a Cheap Radio Control

you will need a drill 12 volt or more
(hobby motor for more speed if wanted)
relay( to change polarity and put 0 volts when o is applied).
car with enough axle room for a drill 
soldering iron and dremel scrap metal
this car was built off of a land rover G4 rc making it easier

1. First take the drill apart and also take the battery so contacts are exposed.

2. Then find the cell mount reqired to steer the car. Solder the same polarity cantact to wire of the cells to the curcuit board.( leaving the old batterry in and, in series will drain the drill battery and charge the lower voltage one.) 

disasemble the gear box and take the gear of the motor so you can drill a hole in the center to the motor diameter. Then cut and drill a metal bracket to hold the motor.  

Then you cut the gearbox away so you can fit the motor to spin a gear using a dremel. Drill holes in the gear box to bolt the brackets on the motor. I cut 1/32 " steel and drilled holes so that I could bolt the motor gear at the right spot.( add tension to the motor so it won't strip the next gear and close the gear box up.

Solder the relay on the motor's wires and solder the relay other prongs switching polarity and on to 2 wires that go from the motor if the relay hasn't had this internally done the. then solder the battery's original contacts to the relay. I can help you with electronics if you show me a photo. This should make your car go about 20 mph.