10D parts including the carburettor are available new from Villiers Services. They have their own website, www/villiersservices.co.uk . The fuel tap is a standard item available from more or less any classic bike firm, probably Villiers Services as well. They will also have the appropriate Villiers 10D spare parts list and owners manual.
There are a variety of on line sources for Excelsior literature, owners handbook, catalogues, and so on. The VMCC library, the National Motorcycle Museum now has the old BMS firm plus Elk
Are you missing the brake shoes or just want them re lined?
Can't help with the fuel cap.
Cheers,
