I have done it (84 bike), no worries. As above; fit the sensor in the drain plug hole of the cooling water pump and run a wire up. You can use the BMW wire/sensor set if you feel rich and run the wire under the tank. In the loom of the early K's there's a 3-lead plug for 'additional instruments' (yes, indeed!), where you can take your power/ground and light signal from. Run a piece of loom from there to the intended location for the gauges and you're done. I have fitted mine (Cooling water temp and Volt) in the left compartment of the fairing.
Cheers, Rene