If your pedal only travels 20 degrees, you can still use a pot that has a travel of 45 degrees (a 1/4 pot), you just need one that would register 5K at 20 degrees. Doing the math that works out to a 22.5K 1/4 pot (don't know if such a thing exists or not, but a 20K pot might be close enough).
Another option would be using a gear set that would yeild a turn of 45 degrees on the smaller gear for every 20 degrees turn on the larger wheel. The larger wheel would need 4.5 times as many teeth as there are on the smaller wheel.
Hope that helps.