Personally, as such a safety critical item, I would be looking for a
new replacement for the entire tank.

Unfortunately, that could take some time, since parts for this
obsolete motor are hard to find. I'm just looking for a quick fix
that's quick, cheap and reasonably secure.

You might be able to find a generic tank and tap a new hole into it (or with
a correctly formed outlet already there). Is the tank some weird and
wonderful shape, or would any old unit do?

Christian.