As a reference, our pre-09 ran a 124F water temp on a 75 degree day during a road race. But, we did not have a sensor in the head. We had it in the hose from the head. Would expect a sensor in the head to read higher. How much higher I don’t know.
Try putting the sensor in boiling water to test the gauge’s accuracy. Check the radiator cap to see if it’s holding pressure. I an not fan of the type of pump belts you’re using and would try a conventional belt. If you do go that route, put on extra belts and tie wrap the spares to the center bearing support. And I’m assuming you are purging the air out of the system before you hit the track.
Another cause could be your chassis set up. If the chassis is too bound up, that will cause the engine to work harder, causing more heat. Look at the gear alignment.
Let us know what kind of RPM’s you’re turning. There should be someone on the Forum that’s run with a head temp. And it might be time to call your local kart shop.
LAD Specialties customer / tony kart / rotax / kt100