The bike in question is a '73 Raleigh Superbe, complete with cottered cranks and 26 TPI bottom bracket threads. So changing cranks is an adventure in itself. I'm not sure how small a chainring I could get on the existing cranks. Plus it has one of the cool old Raleigh chainrings (like this one), and it would be a shame to have to swap that out.

The Sturmey 5-speed looks promising, though. It's available with a 114mm OLD, so no spreading would be needed at all, and the range isn't bad. Plus direct drive is 3rd gear, which will work a lot better on a large-wheeled bike.

I know the SA 8-speed had some quality-control issues at one point. Does anyone know if the 5-speed had similar problems?