Hi, From an earlier post/reply from Mark mbm350, I understand that George Lane can repair them.
Try emailing him at georgelane7ATtiscaliDOTcoDOTuk. and ask the question.
He advertises in ATG, usually inside the back cover.
Regards, George F.
George
350 Sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019