Welcome. I've got a '71 144S with twin Stromberg carburettors. The only real hassle is that the carburettor needles freeze up in winter when the engine (and thus air intake) is cold.

I've also had the temperature problem, and it was due to a water leak that caused an air lock in the top water hose (once the reservoir was empty). I have to squeeze it once the engine is warm to get the air out, as the hose is higher than the radiator. And also have to hold the reservoir and it's hose higher than the radiator. Actually I lift the reservoir higher and lower a number of times, which helps the air through the thin tube. On mine the heat dispersing fins are also disintegrating, but the radiator is much larger than what is required. Try a flush before a replacement, and run water through the system with a garden hose (after removing thermostat).

The thermostat failure mode is to overheat the engine, but this will happen every time the car has run for 5 or 10 minutes.

The engine is all iron, so hopefully you have only blown a gasket. While open, perhaps fit hardened exhaust valves (or perhaps you want to first make sure the engine is still running).

Have fun...