You will need a water temp. sending unit.
It should look something like this.
Look for a port in the water jacket near the head,or exaust port plate, with a plug in it (a threaded plug). You can get a marine type, but an auto-type temp gauge sender that will physicaly fit in this port will work, provided, the gauge is compatable. Auto stores sell them under-dash, after-market temp gauges with the sender. The trick is to make sure it fits before purchase. However, they generally come with a good set of threaded adapters, both metric and SAE. I think this port is on the exaust port plate on yours, but not sure. To make sure you have the right one, crank it up with the plug removed. If ya get wet,you got it