I’ve finally finished replacing the engine switches. Given that it was only the front covers that had deteriorated, for the glow plug and starter switches I decided to simply replace the front covers. These fit on perfectly and it meant I didn’t have to mess with the complex wiring.
I would have done the same with the key switch, but the new front cover wouldn’t screw on. I ended up replacing the key switch with the new one.
As it turned out, I used the B and I terminal posts for this. I had at first used the B and S terminal posts, but nothing happened with that choice when I turned the key. I read some more and decided that the I terminal post was very likely the right one. So I switched the wire from the S terminal post to the I post and it worked.
I also installed a new Guzzler 500D hand pump in the lazarette, which replaces the original one that failed long ago.