You can use a single pot wired in as a voltage divider. Make sure you select a linear taper rather than an audio or log taper. Be sure to measure and verify that, as pot markings are unreliable as they change from country to country. A value in the 20 to 50 kilo ohm range should do the job, you will get a slightly different response range so you might try a few and see what you like best. Be aware that depending on the age of your machine, you might always have full output at max pedal, and the panel control will have no function. (That was the reason for the second pot on the pedal) With lower output machines and an accurate foot, that isn't much of a problem.
Long arc, short arc, heliarc and in-the-dark!