Zack, if you could include in your drawing what you learned about the spring, its measurements, strength, and material, that would be of benefit as well.
Across the pond they have all the available Rootes factory drawings and information, but it looks like they're still sorting through it all since it's a huge task. I don't know if it's "all" the drawings, or just "all that's available" -- and they may not know yet either. Here's a link:
http://www.rootesarchivecentre.org.uk
One of their stated missions is to provide drawings for parts reproduction. I don't know if Speedy was selling OEM or repro items -- if repro, they could just be out of stock. If they were OEM or "the last they're likely to see", then an opportunity may exist...