It has never been explictly stated, but it appears to be either the BMW R80S or the BMW R100S, most likely the later. Unlike Megumi's Bike, I doubt very much that Keiichi's Bike is totally stock, so it is very difficult to place. Understand I know very little about motorcycles, but I did some research on this for an AMG fanfic I'm writing, so here we go...
Most likely it is the R100S, and has parts from the 1977, 1978, and 1980 models (for example, the windshield is definitely from a post 1979, or a custom replacement - it has that little dip at the top).
The sidecar appears to be much more recent than the rest of the car, which would make since, since Keiichi probably didn't add it until about the time he was going off to college. The closest picture I could find was a 1986 sidecar, but it was for the R100S, so it would probably work (I don't know enough about motorcycles to say for sure). Earlier pictures of sidecars I found appear to angular.
His instrument panel also appears custom. His tachometer and spedometer are two different sizes, and from the pictures I've found, they're the same size on both the R80S and R100S. Probably he has to order a new tach, since he probably has the engine tuned for more RPMs.
If anyone out there is a motorhead, and I'm wrong, please correct me.
Larry
22-Feb-2005