Hi Mo
have a tray of old classic bits, but not identified as well as other Brit bits but cant think of any twin gears. can look at the weekend.
just one thing , is it a whine or a screach? a screach could be the bearing about to break up or spinning in the housing, a definate wine probably down to gear noise or bush play.
this is the sort of thing you need
https://www.ebay.co.uk/itm/BSA-SPITFIRE-A65-LIGHTNING-TRANSMISSION-GEAR-GEARS-FRONT-SPROCKET-CASE-NUTS-BOLT/232950778010?hash=item363cf2c89a:g:Z9YAAOSwk3RbtJ7Xwhen I first started mid eighties we had many spares stockists within 20 mile radius, British Iron, Nobby Clarke, L&D, Charlies, Chalky White, Chestermans, Michael Freeman, Chequers and many more, all are gone now. in 50% of them its the children of the original owner not wanting to carry on the business. America (Ebay) seems the best nowadays for used parts. I remember Fluff Brown went to America on holiday , (Fluff owned FB AJS after the factory closed.) he ended up visiting their American stockists and arranged many containers of brand new old stock to be returned. everything from cable ties to complete never raced scramblers.