Don’t have a suggestion as far as what to use for a wire core.
Lacking an old coil were I could get some laminates, I’d making some.
I have a friend with a shear. I’d cut some strips from a steel plate and
file them to fit.
If you cant find another Zephyr coil here is a group of motor that can
donate a coil if you should come across one. This includes the single
cylinder models. They also have a 2 cyl. coil. grounding out one side
to the mag. plate.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.