I'm not sure what kind of thermostat I can use -- two wires, red/beige? and a millivolt thermostat
The Honeywell TS812A-1007 is a heating only, 24 volt thermostat, not a millivolt thermostat. The millivolt one is a different part number. If you have a volt meter, turn the thermostat all the way down and check the voltage across the 2 wire connections; you should get 24 volts. If you don't have a volt meter, look at the gas valve on the furnace. It should tell you the voltage required for that valve. If you need to change the wires to upgrade the thermostat it is not that hard, but you may be better off getting an HVAC guy in since they snake wires all the time.