I have a Monogram Industries gas heater from the 1969 era model # B 1069 Serial # C 17781. Heater has been working fine for 40 Years I turn it on recently and it still worked fine. Pilot light is on but when I turn the termostat up the heater will not turn on. Any Ideas? Other than it's to old

Try bypassing the thermostat. It may have failed.

Usually you'll see a small spark on a mercury thermostat when the contact makes/breaks. Do you see that?