I'd like to point out, having done a bit of study on the subject in a past project, that fin surface area isn't the be-all and end-all of air cooling. Unfortunately, the problem is more complex than that because fin depth and width determines how much air passes through which is also a huge factor in heat transfer.
Bottom line is increasing the number of fins and decreasing fin spacing isn't necessarily a way to improve heat rejection and can reduce it instead.
Surprisingly, the best method to improve cooling appears to be to cowl each cylinder to precisely control the airflow through the fins, since air that passes over the cylinders rather than through the fins isn't doing much cooling at all. You have to do this in conjunction with eliminating cooling leakage though otherwise the general problem remains.
Oh, and crankcase problems? I heard from more than one person that the 912/914 series had crankcase fretting problem at some point during their history too.