Looking at your picture, I don't think that you need to get an original thermostat that closes off the opening at the bottom of the head. It is probably is going to be almost impossible to find a replacement thermostat that matches the diameter of the housing flange, length of the valve closure rod, and the diameter and seal type of the valve closure disk.
I would place a normal cartridge type thermostat between the flanges at the top of the head (see picture of type I am referring to below). The only limitation will be if there is enough clearance in the hose connection housing to accommodate the top portion of the thermostat. Other than that, all you need is the diameter of the flange to match your needs and of course the temperature setting (35 deg C or 95 deg F).
Worth a try!!