Over Christmas a second hand ebay wiper motor arrived so I put it to work driving the crane movement.
I tried to make up a threaded coupling, but couldn't manage to get it straight enough with the rudimentary tools I have here. It turns out the threaded rod I have isn't that straight anyway.
So KISS, and a simple spider drive on the lead-screw.
I brazed a bolt onto a 3/8" nut which attaches to the crane back.
Seems to work.
Only the wiper motor isn't that great. It goes quite slowly and draws a fair bit of power even though it is lightly loaded: 5A at 12V.
The wiper motor is off a Jeep. The Japanese ones I have for the stirrer and winch seem to be much more efficient.
Luckily I only expect to run one wiper motor at a time and my 12V supply for the project is rated to 6A.
I think I'll keep my eye out for another cheep motor just in case.