That sounds like a tough one.

If the spring force can be measured and you think that the problem with the spring is binding and not that it has gotten weak then I would personally search for another spring that is similar enough to work. Often a big old hardware store will have boxes of springs of various sizes, lengths, and stiffness. I've never had much luck re-forming a spring that has been bent and is binding but it might be possible without changing the characteristics of the spring. It seems like yours doesn't want to respond either.

Making a spring isn't that hard but getting it within the correct specs might be hard -I would have to think that it would be simpler to find a spring that is close and modifying it -much be easier than starting from scratch building a spring. Keep us informed about the spring. It will be interesting to see how it all works out.

I almost never see any other hubs than the AW or the occasional older Shimano on vintage bikes so I'm not much help. Seems that the AW is the hub most everyone used back in the day.