The problem with the available radiator hose adapter for a water temperature gauge sensor is that there is no radiator bypass, unless you have a Thermo-Bob installed... You could read engine oil temperature with a temperature sensor in the oil pressure port instead as some have done.
It should be possible to simply tap into the ECU's water temperature sensor on rear of left cylinder head... Let's see what we can find. Here's what I know for now.
Water Temperature Sensor Output Voltage at ECU (5V Input voltage)
Standard: About 2.80 ∼ 2.97 V at 20°C (68°F)
Automotive water temperature sensor output is 180 degrees at 2.938V, and 68 degrees at about 0.65V, so I'm sure onewizard can rig up some adapter for it.
Personally I wouldn't attempt to tie into the ECU temperature sensor unless I was 100% sure that no conflict would occur.There are gauges available that run on 12 volts and have a external RTD ( resistive temperature device)find a place in the back of the motor, jam in some K wool and your set to go. Should be within 5 '