Oh, which one do you ride..?I fly sailplanes and maintain my own motorcycle. Same results, more fun
Well, technically it's oil and air cooled (just like my bike). The problem is that without sufficient air cooling, the oil cooling won't prevent the engine from massively overheating, if you tape over the inlets of a BMW R series with a full cover for example it'll overheat while stationary. Air-cooled motorcycles have absolutely massive cooling drag at speed and that's less than ideal, especially because there are so many good 4-in-line watercooled engines.
The new BMW R1250 might make for a very interesting aircraft conversion. Liquid-cooled 2-piston boxer and capable of 140+ hp at max rpm.
Actually, the best cooling system so far is the one in the ZX10R Kawasaki, it is liquid cooled and oil cooled. And yes, of course, air cooled like all of them are. The 2011 ZX10R produced some 200HP and 210HP with the RAM Air (an airbox model and air intake system), I see some people have even turbocharged the beast for drag racing. The best thing about this engine is it's weight, it is around 60kg dry. Add some 2.4l oil and 1.8l coolant and you get the idea how light it is for it's power.
I have three bikes, one of them is a Yamaha Fazer FZ1S which has a similar R engine, produces only 150HP and weights a bit less than 70kg but it heats so much. At stops it heats balls so much that you really wanna cool off and get to the closest bar to have a cold beer. And it consumes more than the ZX10R or the Suzuki GSX-R which is exactly the base of this Vija engine. So if I would be putting a bike engine into an airframe it would definitely be either the ZX10R or the GSX-R1000. I rode beemers and never liked that junk, hell, I even drive a Beemer and I gotta admit that the Japanese Hondas are better cars when it comes to engines. But the Rotax in bikes like the Beemer GS650 or Aprillia Pegaso which has the same are just junk, having oil leaks, stops often for no reason at all or something goes terribly wrong inside the engine, they are totally unreliable. This is my opinion and experience based on the problems that me and all of my fellow riders had. Maybe it could be a great bke for someone who commutes daily a few kilometers and rides conservatively. The thing is, I want a reliable engine in an aircraft, so I would most probably rule out the Rotaxes or Beemers if I could.
So, chossing between the new 160HP Vija turbocharged and the 120HP Rotax which are both in the same price range I would definitely choose the Vija. Of course, if I don't find a more powerful engine like the ZX10R or similar. High power in a small box.