I have the same Synchrowave machine. In a quest to fit a small ESAB pedal to my Maxstar200, I took apart and studied the Synchrowave pedal. Very heavy duty with a big wire wound potentiometer. There are only five wires regardless of the connector used on most old machines. Two for the on/off switch. and three for the potentiometer. Miller has the schematic online in the manual. On mine A,B are the switch and C,D,E are the pot. It seems that the pot value is not that critical. 1000 to 10,000 ohms. Just make sure it goes to 0 ohms with the pedal full down. Also some pedals have capacitors across the pot. If you have a newer machine with lots of "smarts" you may want the caps.
Mating connectors are available on ebay or from Miller (10X $$)
There is a lot of DIY TIG pedals on You tube. Also a cool Pulse attachment.
https://www.youtube.com/watch?v=hk02102nefk
https://www.youtube.com/watch?v=-P6w-tj8_Tk