that is called an MSC, manual speed controller, it was pretty much a servo that swiped the servo arm across resistors to add more resistance to go slow and full throttle only be across the last resistor causing least resistance, they are very old (old as far as RC's go anyway), 80's-early 90's old (saying thats old is making me feel old now...). they are very inefficient as its always drawing the same current as full throttle but only using a portion of it.
that is an awesome deal for both at 100$ what was unfortunate?
as far as spare parts, id find a manual to get part numbers and trry to find them that way, given that it is a tamiya though and a pretty popular one at that in the day, there are probably a few aftermarket part manufacturers for it.