For what it's worth, around 7-8 years ago my Lyman Mould Master XX 20# pot that I bought new in 1979 and had cast 1000's of lead projectiles with 'went cold'. The thermostat went bad, so I contacted Lyman about getting a new one. Since they no longer market that model, they advised they no longer have parts on hand. They gave me a phone number of a business in Missouri that they give out for replacement thermostats. I called and talked at some length with the owner who advised that the thermostat he sold was in fact better than the original Lyman ones, but required a little tinkering to make them work. The guy said the thermo he sells allows a user to control the temperature more accurately and is built better, advised he has sold quite a few for Lyman pots. For around $43.00 I got a new one, plus he sent me a diagram on how to hook it up. Nice guy, very helpful. 'Hooked' the new thermo up, all required was drilling several holes in the housing that you attach the thermo to and connecting the wires. Has worked 'as good as new' since, someday I'll have to replace it entirely, but for the present, I'm a happy caster. If anyone has a MM XX with a bad thermo and need to replace, PM me, I'll give ya the number. See that a used Lyman MM XX just sold on Ebay for around $110.
Note: Talked about this repair once before on the Darkside-can be currently found on page 34, towards the top.