Ancient Diana Needs Help

Sounds like a fun project.

Chambers carries some parts for the D27 if you don't mind ordering form the U.K.
https://www.gunspares.co.uk/products/24376/27/

Air Rifle Headquarters can make a custom spring if they have nothing compatible in stock. Just give them the specs of the old spring.
http://www.airrifleheadquarters.com/spring-matching-service.html

Vortek can hook you up with a custom spring also.
http://vortekproducts.com/ourstore/index.php?route=product/category&path=64

Good luck and let us know how it turns out.