I've had my Mag 20 go down a few times, usually because of a break in the heating element. At one time Lymans Service Department would sell you parts, since all the new stuff is from China anymore, I kind of doubt they would do that anymore, they wouldn't even sell me replacement handles for ingot moulds. My last MAG 20 failure was the thermostat, which it sounds like could be your pots issue, and it was a total loss. I have cleaned the contacts in the past to get it working again, but it didn't work that last time. I called Lyman and was told to send it in, and the price was around a hundred dollars plus shipping. With shipping both ways it would have been around $150.00 or so.
So I bought a PID controller https://www.omega.com/en-us/resources/pid-controllers, actually I made my own and eliminated the factory bimetal controller that really didn't work well to begin with. I hotwired the pot to be full on, and controlled the temp with the PID and a thermocouple. This has lasted 7-8 years so far with only an occasional thermocouple replacement. If you're not comfortable working with electricity I'd suggest going the Lyman route. Rewiring pots and building PIDs isn't everyones cup of tea. I think I ended up having around $60.00 in building my PID, plus an inexpensive thermocouple. You can also buy premade PIDs from Auber Instruments https://www.auberins.com/ There are also other options for PIDs, I think a couple of guys on the boolits site are selling them as well.But please verify that this is the issure before spending any money of course.