Hi John
It is quite normal for the PilotAware Classic unit to get warm. Don't be alarmed. The main sources of heat are the RTL dongle and possibly the wifi dongle.
Be aware though that if you put your PilotAware Classic on the cowling in full sunlight the solar gain will be greater than the power dissipated through the electronics.
PilotAware takes about 2A at 5Volts so 10Watts. As it doesn't move, and only emits a small amount of light the majority of this power is dissipated in transmitted power and heat.
Keith