Head temperature is one of the tuning tools available to you. It rates last on the list. Air cooled engines are affected by the outside air temperature and the amount of wind flowing over it. A sensor goes under the spark plug to read head temperature. The correct temperature on a cold day is different than a hot day.
Next would be spark plug color. Still a very rough way to tune.
Exhaust gas temperature tells you the heat of combustion, and it is directly related to air/fuel ratio. Unfortunately, as you lean out the mixture and the temperature gets hotter, you reach the peak temperature and leaning it out more drops the temperature. You can be fooled by being on the wrong side of the curve.
A wide band oxygen sensor will tell you how much oxygen is in the exhaust. Excess oxygen indicates a lean mixture, excess fuel indicates a rich mixture. A sensor, cable and gauge will set you back $200. It is the only true way to tune.
Because the amount of air entering the engine is highly variable depending on altitude, temperature and humidity, you really need an active monitor to be able to react to changing conditions. Changing the jets between runs may be required if outside conditions change.