But the really brilliant part is that it looks like I can test the chip mostly in isolation. Just need to give it 5V and a couple clock signals and it should run.
And thanks to the excellent description of the chip's internals, I should be able to check most if not all of its functions.
So my plan now is to attempt that. It's a first for me, but I feel I have accumulated just enough knowledge and experience to pull it off.
If it works, I know I can fix the entire machine.