Some of those parts you need will probably be compatible with the e12 or e28 Fives, depending on the year of your car. Check if so, cause you may then find them easier. Just a thought. Maybe also ask Clownshoe if he has any contacts, as he bought one of these a little while back. Good luck with your search.
Like Sith says many parts are compatible with the e12 (pre 82 635s) or e28 (post 82). There is a slightly more complicated engine evolution though. Suspension is the same, some interior switches. Also e30 window switches (upgrade to illuminated switches) are the same aswell as door levers. The fuel sender should be the same as any period 3.5l (or 3.3l). although maybe not. So most scrap yards should have what you are looking for. Take the part number with you.
Windscreen... no idea sorry. Although there are some 6ers being broken up in the junkmail.
Light clusters your best bet is Ebay.
and try http://bigcoupe.com/ as the onestop 635csi authority. Do plenty of searches before posting questions, chances are it has been asked before.... its a bunch of grump old ballies that hate answering the same questions every week