As much as you dont want to hear it, you are much better off rebuilding that part
of your pedal design.
I had the same issue with a PC pot in a custom starwars yoke.
I found a package of green gears at a local hobby shop which
solved the problems (with some drilling of the centers).
If you do not wish to use gears, you can also use a system of
levers to create the mechanical leverages needed. Do a
search on Pedals, or Sim racing, and you should see many
great examples of this.
The more control ability you have in Spy HUnter, the better !
Also, you may find other games do not react well with
a reduced value pot. Everything from windows
calibration itself... to win games, and even other games
inside mame... all may exibit poor control or lack of
functionality. Even failure to operate properly, if at all.