Thanks Kurt!
Now I'm thinking about changing those big 26 x 1 3/4 x 2 (54-571) tires. The originals were probably 26 x 1 5/8 x 1 1/2 (40-571) but they are hard to find. 40-584 are available, but they won't fit. So I could:
a) change rims to 584 and use 26 x 1 1/2 (40-584)
b) change rims to 635 and use 28 x 1 1/2 tires, which are available and I'd prefer. But I'm not sure about it because clearance would be very tight and probably brake blocks will need an adapter plate
c) leave it as is
A local LBS has a deposit with some old stuff in it. I'll try that...