You are probably too far in for it to be helpful but it may be worth just getting them to fix the vacuum unit and then order a new electronic unit. Summit Racing in the States seems to have several options:
https://www.summitracing.com/search/part
type/distributors/make/chrysler/engine-size/5-2l-318
I have read that distributor in the A series Poly 318 as fitted to the Bristol is interchangeable with that from the later LA. That said, I'm not an expert. It's just a suggestion to speed up your research. Best of luck and please report back.