Just any old 50k single gang pot of the same physical size should be fine. They're all pretty much the same and only a few dollars anyway. Don't know if it's a linear or logarithmic, try the linear first (either will work but one will change speed at a weird rate).
I would be very surprised if the pot was connected directly to the motor coils.
Last edited by neftaly; 08-14-2013 at 05:02 PM.