I purchased a pedal go cart back in 2005 from a car boot sale and then made a few modifications to it.  I strengthened the frame and axles, put stronger wheels on, removed the pedals and fitted an electric motor from a Sinclair C5 car.  The power to the wheels is fed through a gearbox taken from an electric angle grinder and then through a sprocket and chain set taken from a petrol lawnmower. 

I also designed and built a microprocessor circuit based on a microchip PIC 16F877 and some power FETs for controlling the speed of the motor using pulse width modulation.  The microprocessor also controls the 4 headlights fitted to the front and the 2 red lights fitted to the rear of the cart.  The speed of the cart is controlled by a foot operated throttle pedal that turned a variable resistor that is connected to the microprocessor.

As can be seen in the video below, the cart is capable of a fair speed for a child.