I talked to the spring makers, and they agreed to custom-make a spring based on my original if I sent it to them. So I boxed it up and sent it, along with the incorrect new one that I had received.
I brazed one spot on the backrest spring where a sheet metal strip had broken, cleaned the spring up, and spray-painted it black. The weather has been especially freezing, so the paint (which was supposed to dry in 12 minutes) took all day to dry.
I mixed up a second batch of epoxy and continued filling in cracks in the steering wheel. It’s rather tedious and messy work, and there are a lot of cracks to fill. It will probably take several more sessions to finish the job. After that I will need to sand it all smooth and touch up any places I missed.