That does sound like it is the pot that is the problem.
Now, if you can't make out the rating, easiest way would probably be to short the wires to the switch and try a few cheap regular pots and try to find a value that works well. If too low it won't reduce speed (much), too high it won't start (until maybe at the end). Once you found one that works well, try and locate a replacement with the required switch. Be careful though.