Where Is the Temp sender unit.

We  have an F reg clipper 1.8 carb engine but the temp gauge is not working at all. In the engine bay the is a disconnected wire coming out of the harness just next to the oil pressure switch on the head. There dosent appear to be the small brass sender unit for the wire to go onto. The maual says it should be in the heat exchanger hose ? but no sign of it Can any one tell me where it should be.
If the engine has been changed at any point how do you tell what code the engine is? the block says 1.8 cast onto it but is there still different codes.