Hello David
No chance for fixing the engine?
Have you tried at Pius Kaufmann? He has really lots of spare parts, but I am not sure, if will sell them... he's a bit of a hoarder.
And there is Henry Hässig from britishparts.ch (Flawil), he might be of some help. Or Marcel Baschung (classic-bikes) in Niederbipp. These two I know...
Other adresses are eicher-classic.ch in Unterägeri, Richard Zürcher (british bike service) in Neuheim, ZG, but I do not know them.
God luck!