New shafts are readily available.

Do you want web-links to companies that "re-hab" the assembly and/or "rockers"?
http://www.rockerarms.com/pages/gm.html

Chevs of 40s sells a re-habbed assembly, with your core.
https://www.chevsofthe40s.com/dept/Engine/cat/Cylinder+Head.html

Kanter & Egge used to sell rocker arm assemblies/parts.