Most air cooled engines have fins which are set at an angle - not completely horizontal. That's because back in the days when air cooling was normal the designers knew the bikes would be stationary sometimes, and so the fins allow convection circulation. (warm air goes up past the fins because they are tilted upwards - go and look at an air-cooled engine and you will see what I mean)
I've only ever had one watercooled bike - keep it simple! When water cooling goes wrong it's a real pain, and on a small petrol engine it's just not necessary...
Tony
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