Just a quick update: extracting the spring was quite easy. Using leather gloves (and eye protection, of course) and starting at the broken arbor end, I pulled it out half-a-turn at a time, holding the unwound part into the case with a finger.

The spring is 20 feet long, 0.015 inches thick.

For the curious, I've attached pictures of the spring and case, the case with arbor and side attachment points, and the tempered end of the spring.

I won't be able to do anymore on this project until next week (or maybe next month), but I'll let everyone know how it progresses.