it is a sensor on the output shaft, it's just a little magnetic sensor that is triggered via the gear teeth flying past it
much like this:
http://www.cherrycorp.com/english/se...007_Series.pdf
I'll see if I can find a part number for you Friday, until then I won't have time to get to the books or be near them otherwise
NOTE: the linked spec sheet is a similar device, I do not know if it would work or otherwise and I don't recommend trying it because I don't know what voltage and amperage the stock sensor runs at