This is what I used.
Racing Digital Temperature Controller Sensor Engine Alarm Oil Water Gauge Meter | eBay
it reads In centigrade so I had to get used to that but I put it where I can see it while driving the boat. It doesn't have great daylight visibility.
Be sure to connect the thermocouple to the block where there is a water jacket. The ends of the head are a good place.
These meters have relays for max and min temp so you can attach a separate buzzer or light or both. they do have a built in alarm, but installed in the panel that I built the plastic cover muffled the sound enough that I decided to install another sound device.
I set mine to sound at 90 C which is around 192 F.
If you keep your engine from over heating it will pretty much last forever. Virtually all major problems these engines have can be traced back to an over heat, Head gaskets, loss of compression due to cylinder scoring, leaking sleeve seals, water in the oil, oil in the water.
