Alrighty then- From an American point of view. I think that to me, there are NO "rules" for defining a cafe racer. I think the machine is secondary to the idea. You're taking something that you can afford, and doing what you can to make it both perform better and be more to your individual liking. Sometimes you can afford something downright racy, sometimes you can't. When I built my Suzuki GT380, I did not intend to build a "cafe racer." At the time, I'd barely even hear the term! I just knew what I liked, and got as close to that as I could. The big thing to me was, I invested heavily in those things I considered REALLY important. I laced a 17" rim to the stock front hub, I bought a box-section swingarm off a later bike, I replaced the points with an electronic ignition, I bought expansion chambers. That's it. Everything else was either home-made or second- (or third or fourth) hand. But I loved it, and even though it's long gone, I still do.
Yes, I ramble a bit, but the point is, it doesn't matter what you start with.It doesn't matter if you mix eras. Cafe racers are about individuality. Rebellion. I'm getting older, but I still have that rebellious attitude of my teenage years in my heart, and cafe racers are the ultimate form of flipping off the established norm. I admit I cringe sometimes when someone puts clubmans on a bike and says it's a cafe racer, but that's because of the labor involved- the bond you form with the machine. If that's all the work you want to do, what bond do you have? When you take your bike down to the last nut and bolt, exchange parts, explore its mechanical innards, and re arrange it's organs as you re-assemble it into YOUR vision. THAT is my idea of a cafe racer.
