I've emailed all the suppliers of classic parts I can think of and if nothing turns up I'll make one. After a good soak with oil it's now producing a heady 0.7fpe with cat slugs and I think that's as good as I am going to get. Waisted pellets don't exit until the second try, even after being seated in 10mm.
Not tried it with darts yet as it's still spitting a little oil after about 50 shots. A rub with turkey foil and a touch of Philips gun blue paste has sorted the rust spots on the barrel so It's almost good to go.