Thanks for all the suggestions. There is nowhere near enough rust to
make reassembly a problem. There's just a bit around the edges and some
dust between the plates. I would try electrolytic de-rusting, except
that I'm concerned that it may remove the intentional green oxide layer
(it is an oxide layer and not varnish in this case). I'll give the
laminations a wipe with white spirit before I reassemble. Before that I
need to build some kind of jig so that I can wind neat coils. It didn't
occur to me at first that using thick (2.5 mm) wire would make winding
more difficult, but it looks like I need to avoid twisting the wire as
it is transferred from the reel to the transformer bobbin. I'll post
some pictures when it's done (probably a week or two).

Chris