So my 'main' IBM Model M keyboard died last year - some of the traces on the membrane have corroded, leaving the keyboard pretty useless. I have a spare donor keyboard which I can use to repair my 'main' keyboard... Replacing the membrane on a Model M keyboard is not a simple task... This first week is a bit of double feature, since both of these videos really do go together - I just didn't want to put out a 40 minute video! Part 1: Part 2:
The membrane looks repairable with conductive ink. You could then protect it from further damage with conformal coating, or just varnish (being careful that the solvent in it doesn't dissolve the membrane).
I like seeing old things maintained to keep them working as long as possible. I hope you get it working fully again, however long it takes. In part 3, I saw "A2" on the bag of bolts, then you mentioned a magnetic screwdriver and I knew what was coming, because of course that's A2 stainless steel, which is only weakly magnetic, so it won't stick to a screwdriver. Then in part 4, I was screaming internally after seeing a spring misaligned on the far left at 8:50 which you didn't notice and installed the rubber sheet over. Have fun taking it apart again!