Fastest motor: 4.2pro, from toyeast.com or at ebay.com (seller: toyeast) from Kowloon, Hong Kong. Many of the users here including myself, have ordered from them with no problem.

If you don't want to wait for the 2 week delivery time, the walmart bit clone called Motorworks has a hardwired 4.2 motor with plenty of torque. Also, the knobby wheels of the Motorworks clone (with the shorter wheelbase similar to Microsizers) provide the best traction for your mini rc. Unfortunately, you will have to solder the wires from the hardwired motor to the motor contact wires of your zipzap. This shouldn't be a problem if you are handy with a soldering gun.