If none of this does the trick, drain the coolant. Remove the thermostat, and check it and the radiator cap. The thermostat should open at the temperature specified in the manual, and the cap should hold the specified pressure. Replace either if it’s not up to spec. Fill the cooling system with fresh coolant, and make sure you purge the air from the system. If your bike still runs hot according to the gauge, the gauge or the gauge sensor might be faulty. Finally, a bike this old might have blocked passages in the radiator, hoses, or water jacket, which could mean a complete teardown.