Hi,
If you are a member of the LDMCC, you can get gaskets and cables from the post war spares scheme. Leightons in Birmingham, UK can help out with a dualseat. There are also several suppliers of wiring harnesses advertising in 'The Old Bike Mart'.
By the way, if you listed your location, we would be able to give more relevant advise - after all, this is a World wide forum and there is little point in suggesting a small supplier to an owner on the other side of our planet!
Regards,
Eddie.