So. I found a reasonably priced 3GM30F on eBay that was in better condition than Raasay's present installation. It also has a heat exchanger, so it might be less rusty inside ...
I saw a video of it running before I bought it. It's presently sitting on a pallet in the cellar.
I've planned the swap. There are several stages to this, starting with removing the old engine.
I was going to disconnect it and record everything about that process today, but when I arrived at the boat, I found that the domestic batteries were flat. I couldn't figure out why, at first, but then realised that the bilge pump must have been running a lot and draining them faster than the solar panel could charge them at this time of year.
I haven't figured out where the extra water was coming from.
My NOCO Genius 26000 charger doesn't like my current batteries for some reason (they are AGMs, but they are an American spec, which might make a difference). I've tried various things, but haven't found a reliable solution yet. In any case, it wouldn't look at charging them from flat.
After some experimenting, I'm using the old engine to charge them. The Sterling charger seems to be better able to control them than the NOCO charger. It will take a while, though...
And I obviously can't get far with disconnecting it while it's running. So.
I think I'll take the starting battery and the air filter home (the new engine has lost its air filter) and try the new engine out, rather than disconnecting the old one.
You might wonder why I wasn't going to test the new engine first in any case - I'm now wondering that myself. Maybe I was too reassured by the video ...
In the meantime, the batteries seem to be charging (although being really flat for a few weeks will probably have damaged them) and I can switch the little mains charger for the starter battery onto the house batteries to make sure they don't go flat again. It's only a 5amp charger, but I don't think the bilge pump should use 120 amp hours a day ... maybe. Have I got the maths right?
I hope it will be safe to leave it on the shore power until I get another chance to visit. Which will have to be soon.
I had planned to make a start two or three weeks ago, but then I fell ill. It all needs to be done and dusted by March. Which now seems very close.
