For me it has to be rim brakes, and 90 to 96 I guess. Doesn't need to be factory spec, but has to be done with bits available then. I upgraded my bikes back then, and see nothing wrong with tricking out a retro ride. In fact I'd go as far as say that they are more true than the factory spec bikes, because we were all tricking out and personalising our bikes at the time, at least all my mates were. If you had a 100/200GS 300/400LX bike, and I did, the first thing you did was start upgrading the groupset.