Disappointingly, the R80 tank hasn't materialised.
While I was out and about, I did manage to buy a couple of lengths of bar which meant that I had some material to maching the crush tubes for the new rear engine mounts out of. I also bought some 1 1/2" "exhaust" tube.
Some careful calculations showed that by making the exhaust for the BMW from 1 1/2" OD 1/16" wall tube, I would have a former to bend it, and the gas would travel from one end of it, to the other end of it over a period of time. Quite what it's going to do in terms of performance, is a bit of a mystery, but it will almost certainly rob the midrange of something. Given that the later bikes needed bigger valves to make more power, I suspect that a fatter bore exhaust isn't going to tap any unsuspected reserve of horsepower either.