Thank you all for your replies. One of the reasons I am considering the ContiClassicAttack is because radials don't grow as much at speed as do bias-ply tires. The main reason, though, is that they are supposedly the best-handling of all tires narrow enough for older bikes, at least according to the March 2017 issue of the German magazine Motorrad Classic. Here are their test comparison results with point scores:
8th place, 156/250 -- Avon Road Rider
7th place, 167/250 -- Pirelli Sport Demon
6th place, 174/250 -- Metzler Sportek Klassik
4th place, 200/250 -- Bridgestone Battlax BT45
4th place, 200/250 -- Continental ContiGo!
3rd place, 201/250 -- Dunlop Arrowmax Streetsmart
2nd place, 204/250 -- Michelin Pilot Activ
1st place, 224/250 -- Conti RoadAttack 2 CR/ClassicAttack
(The 1st place RoadAttack happens to be in their "CR" DOT-approved race compoind, so it's a bit of a ringer.) The narrowest front RoadAttack, 100/90-18, requires a minimum rim width of 2.50", which is why I was looking at the 90/90-18 ClassicAttack. Thank you very much, JCW, for the PDF of the Conti catalog. As I assume you discovered, it is no longer on Continental's website and is much more comprehensive than the catalog currently posted. It is a tad confusing, however, in that they show the 90/90 as also requiring a 2.50 minimum rim width whereas their 110/90-18 is seemingly suitable for a 2.15" rim - go figure. Incidentally, they show the centrifigul growth as a mere 7mm on the 90/90, so that's good. I'm obviously going to have to read more in the Conti catalog before making any decisions.
I was definitely incorrect about Avon being the sole company that lists actual tire dimensions, since I discovered that Bridgestone also does so, and that Conti once did. That said, it's obvious that the manufacturers generally don't cater to custom builders, who need to know exact dimensions of all sorts of things. I haven't added anything to my build thread in years, but anyone who cares to take a look at it can google "T140 cafe project, cont. britbike".