I've got round to my Twenty/Shopper and besides replacing the tyre also went ahead and got a new KMC chain. Of course, the links were somewhat shorter, and so I couldn't cut it just right without having to adjust the rear wheel drop-out. In the end, this is what I got.
Is this drop-out position safe to ride? The nuts are secured to about 300 inch-pounds
UPD: I was just reminded KMC sells half-links, so might just get one and a missing link to lengthen the chain a bit. (now I just need to find a K710 missing link somewhere on this continent)