Good news: I asked on SteveHoffman.tv about the mono track of the new YS release, and apparently it is 100% original mono. (And it's lossless to boot!)
The only thing that needs to be found now is a mono copy of the U.S. cut. It did air on TV in the 80s, I think. Even a faded 16mm print would work, as long as the sound is in good shape...