About 12 years ago I bought MAC headers and mufflers for an R90S - links below - very well made, chromed, sound great, somewhat more throaty than stock, but not loud. I owned and rode the bike for 5 or 7 years, sold it to a good friend. I recently bought the bike back from that friend after he also rode it for a good number of miles. The MAC exhaust system still looks perfect and sounds the same. Not cheap these days, but I can recommend it highly. Good luck to you.
http://shop.macperformance.com/MAC-BMW-Twin-1970-1984-Chrome-Replacement-Head-Pipes-181792-D.htm?categoryId=-1
http://shop.macperformance.com/MAC-BMW-Twin-1970-1984-Replacement-Whispertone-Mufflers-105627.htm?categoryId=-1

Thanks for the links!
Much appreciated
cheers,
-Marcus

Duane Ausherman's website has a fairly comprehensive section on aftermarket exhaust. Definitely worth checking out imo

http://www.webring.org/l/rd?ring=bmwbikering;id=136;url=http%3A%2F%2Fw6rec%2Ecom%2Fduane%2Fbmw%2Findex%2Ehtm