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Vaguely related, I recently found a study of the pro's & con's of air cooling. interesting points were:
On a volume basis, you require 4,000 times the flow of air compared to water (I appreciate that eventually you will need to duct that air over a radiator if you use water cooling, but you can tailor flow through the radiator whereas it must be compromised over the engine).
However, the air adds nothing to the weight of the car. You can expect a weight saving around 10lb (5kg) per litre of engine size.
The finning around the barrels means a much longer engine (expect cylinder spacing not less than 1.4x bore size). A 3litre engine might need to be 8" longer. Not only does this add block weight, but vehicle layout may be compromised, and most crucially an 8" longer crankshaft is likely to be more fragile and susceptible to torsional vibration resonance.
Care must be taken with finning around the head to avoid heat distortion (with the inlet side likely to run naturally cooler than the exhaust side).
The air-cooled engine is less likely to be compromised by light damage (a stone hitting the fins is rather less of an issue that hitting an radiator), and fewer (or entirely removed) risks relating to the fan, fan belt, coolant leaks, dodgy thermostats, etc. In short, once you've got the design right, it will be almost bombproof.
At the same power output, air cooling is unable to remove heat in detail areas as efficiently. So the combustion chamber surfaces run hotter (bad for combustion, more risk of pre-igintion limiting performance), as do valve seats, rings and top-end pin (increasing the risk of failure) and piston face (higher risk of burning a piston)
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