Best to use new crush washers if possible. Any good brake shop can supply you with some copper washers which you can reuse after annealing. If you real careful, you can reuse the aluminum ones.
Cover plate O ring will last many a mile.
If you choose to reuse a OEM crush washer, do NOT press your luck with multiples. After two cycles,time to be a un cheapskate and buy new. Telling you, the copper rings are GREAT, heat with a torch to cherry red, quench and your good to go.
Here's a link for annealing aluminum washers. I have
NEVER tried this so your on your own. However, stuck in a remote location without a nearby BMW dealer, worth a stab.
http://www.motorcycle.co.uk/reference-material/annealling-copper---aluminium-washers.aspx~~~~~~~~~~~~~~~~~~~~~~~
The issue with re-use relates to their softness and the fact that if the two surfaces that first compressed them left irregularities in the aluminum washer's surface then, if it doesn't line up just as it had in the first compression, you'll not have as good a seal.
I prefer copper myself, but that's become pricey and not used as oftern (I think largely because of cost). Copper can be annealed between uses and even re-formed if necessary.
If you are caught having to reuse the aluminum washers, then it can be useful to inspect them carefully and, if they show signs of raised ridges, you can dress them flat on some emerycloth on a flat surface, clean them off, and re-use.
Aluminum can be annealed too, it's just a lttle more difficult to do without melting the washer. A smear of soap or washing up liquid on the washer will turn black when the correct temperature is reached.