An easy method is to use a thermostat from a portable space heater. You might need to position it a certain distance from your heated brick, so it responds at its temperature range.
Or install a relay, controlled by an old-fashioned room thermostat. Position the thermostat a foot or two above your heated brick. Assuming the air is calm, air rising from the brick can assume a normal temperature range for the thermostat to turn on and off.
If you search you might find another temperature sensor, such as automotive types, radiator temperature, etc.