hi

Correct thermostat, but i would also check that the system isn’t full of crap, try back flushing the heater matrix and cooling system and refilling with anti-freeze, once you have done all this and the temp gauge is still playing up i would have a look at the fan housing more over the clutch inside it.

now i am not sure with the newer fourtraks if you have the same set up as mine i have a constant engaged fan, that meaning its running all the time, I am unsure if the newer fourtraks have this or a temperature controlled fan.

This is a normally rock solid engine but the head and block are made of cast iron and if antifreeze isn’t used you can get a rust build up inside so anti freeze is a must, when I did mine a found a lovely brown sludge