The VW air-cooled engines that I build for ice machines and for street have a lowered compression ratio to help keep the head temps where they belong. The other things to prevent the air-cooled engine from thermal overload include making sure the tin work is in place, including thermostat and linkage. Also, the fan should draw in fresh air that has not been through the engine s cooling system, or that has passed by or through the exhaust system. In other words, the coolest air possible. Scoops and duct work can help a great deal in getting the coolest air to the fan.
Timing is also a large factor in engine temps.
As little as 10 degrees off in timing can melt pistons and decrease performance. Also frequent oil changes can keep the internal friction to a minimum. I have seen engine compartment temps influence fuel and intake manifold temps and the dramatic drop in the horse power readings on the dynamometer as these temps go up. Never put the gas pedal to the metal on the hills, especially on those hot summer days. Without doing this the engine will easily be pushed to the point of pre-ignition.