Regarding air vs. liquid cooling, I have noticed that a number of manufacturers have produced modern, short-stroke, liquid cooled singles that turn some amazing rpms and seem to practically duplicate or surpass the performance of the similar displacement twins of yesteryear. Not necessarily fact, just my observation. BMW and Aprilia have a 650 single, for example. And I notice that most of these modern, CVT scooters, at least those with 200cc+ displacement, are mostlly liquid cooled and have amazing performance, especially when one considers their considerable weight.
I think liquid cooling is becoming more commonplace because they have leaned out engines so much in an effort to reduce pollution that they run hotter than before.