Some old news, but a while back, I've had the luck to find an original Kei-Hin CR29 carb on ebay in Germany.
The seller didn't know what he had, what meant I got a bargain.
And after some searching, I found an intakerubber that will fit it, comes from a CBX1000. Ordered it from the local honda dealer, so it is fresh rubber. No problems with carbs falling off, as we had with old imitation manifolds on the CB50 racebikes.