Welcome to the Horde of the Archaic! I'm just sorry these electrical woes seems to have driven you here.
It seems our esteemed "Mr. Magneto", Adrian II, has already placed some solutions on the table for you. You could also try reaching out to our Forum hosts, Hitchcocks in England at
https://accessories.hitchcocksmotorcycles.com/. They should also be able to get you sorted out. I believe they sell just the Boyer boxes to suit your situation like this one:
https://accessories.hitchcocksmotorcycles.com/19155, but it might be best to just give them a holler first.
Like yours, mine also has an electronic unit fitted by a previous owner, some "no-name" one that hasn't given me any trouble, but going back to the original points setup, as Adrian II suggested, wouldn't be the worst idea, might be a bit cheaper, and still do the job well. Did you get any "old parts" with the bike? 'Cause what you need might just be rattling around in the bottom of that box.
And let's see a few pix of your project!
How Adrian II might appear getting ready for a nice ride