First of all you can't use the sensor on the head for two purposes. You must have 2 sensors, one for the temp gauge and one for the fan. Secondly, the fan will require a thermocouple type sensor which operates a relay to switch on the fan. Thirdly, the fan is there to cool the water in the radiator and the sensor should be fitted on the rad or at least in one of the pipes connected to the rad, usually the top hose. If you take the temperature behind the thermostat the fan will switch on before any hot water reaches the radiator.
https://www.ebay.co.uk/itm/AeroLine-Adj ... SwHjNV8Tt9This is the type I've used in the past and it works very well. The thremocouple tube is placed inside the top hose at the joint with the radiator and the joint sealed with some silicone gasket sealer. You will need a relay wired in as well, this will operate the relay for you.
Or, you could mount the fan on the front of the radiator as a pusher type fan, retain the existing fan and operate the electric fan manually with a switch when you see the temp gauge rising too high.
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