Icharus,
All of the Century high freq boxes I've worked on in the last thirty years
have had a fingertip control gas valve in the handle (factory model).
Never seen one with a amp pedal; most people set the jack plugs or shunt as close as they can and go for it, as has been covered on this forum a few days ago. You might try Lincoln Electric/WCTA tech service at 86.236.0044, option 3. Good luck!