I finally got my C128 up and running. I had to buy another one, which I got cheap because it was known not to boot. After swapping out some parts, I got everything going perfectly., including replacing the missing key. I ran the dead test diagnostics on for a couple of hours and everything, including sound passes.
My next project is a Sinclair 1000, which does not power on at all. I put the power supply on a multi meter and it seems to be working to specification, so the issue is the machine itself. I opened it up and there is no obvious point of failure. I am thinking this may make a decent Raspberry Pi based Cyberdeck, the main challenge being adapting the keyboard to work with the RPi.