I don't know of a replacement part but I have been pleasantly surprised how helpful my local auto-electrics rebuild place has been on other projects. If you take your unit to one of these guys they may well be able to help. Or tell you there's no chance! They're usually on some anonymous industrial estate well hidden away, so might need a search. Maybe I was lucky but I get the impression these guys would rather repair than throw away, proper old fashioned.
REgards, Mark