<Puts his refurbishment hat on>
My clamshell upgrade had a similar problem. 2 evenings with various wire brushes / emery paper etc and what looked like a heap of rust turned out to be a reasonably solid case underneath.
And mine didnlt seat either (tiny section O- ring so the slightest thing and it unseated)
Resprayed it with etch primer etc. Seems to work OK.
Anyhow, if that doesn't appeal, or it has actually rusted beyond repair, the places I have had success with are:
David Anderson Marine (Tayport)
Ardoran MArine (nr Oban, Suz dealer but does get old stuff as well)
MJ marine (New Holland, by the Humber)
Boating Mania in Upminster (but that was a few years ago, so may not still keep the old stuff)
And there's a place at Pembrey or Bury Port (Whichever has the Marina) that I can't remmeber the name.
All findable on Google. I should of course Mention Bill Higham in Manchester who has a huge collection of old stuff, but opinion on here seems variable as to his pricing policy.