You should be able to use the derestrictor in the thread below, like I explained in it. You don't need anything other than the Arduino PCB (about £3) - no battery, switch, transistor,coil, etc. You find the speed sensor wire from the motor, cut it and join the motor side to pin 2 and the controller side to pin 12. Use any 5v and ground from the controller to power it.
http://www.pedelecs.co.uk/forum/threads/ebike-derestrictor.25266/
If you're lucky, you'll be able to install it inside the controller box, so no complicated wiring.
If you want to be really clever, run a bridge through a reed switch and diode between pins 2 and 12 so that you can activate/deactivate it with a magnet.
