Your 15-91 likely needs the dual lead 3 prong plug, one lead goes to the wall plug, the other lead to the foot control. If you don't want to do any rewiring, buy the dual lead 3 prong plug with the foot control already wired. I'm not sure why there would be a single speed foot control, if you're going to sell it I'm guessing most buyers would expect variable speed capabilities.
Assuming no one has changed the wiring on the back side of the 3 prong receptacle on the machine then it should be plug and play.