Many parts are still "get-able", but some are not. And, even some of the "get-able" ones are becoming quite pricey. Some items are small and are VERY handy/important to carry as spares in the bottom of a pannier on long trips, so with these thoughts in mind:
(Verify these things are OK before packing them and counting on them to save you or someone else while on the road):
ICU
diode board
Spark plug wire
Clutch Cable
Voltage regulator
Fuses
For bikes using a "splitter" cable setup for choke/throttle, the longer of the "short" lower cables - could be used in a pinch to replace any of the (4) cables whose ends might break.
Headlamp/starter relays.
That's the "cheap but get-able" stuff...
The expensive or harder to get stuff:
Transmission (even if not 100% perfect, you can get it fixed up in your spare time and have a replacement to fit any airhead from 1981 ->1995.
Rotor/stator (brushes are cheap).
Heads (early 80-s bikes will need valves&seats redone at some point, having a spare set of heads allows you to keep riding while getting the other set fixed up.
Carbs (again, having a spare set to work on/swap out as needed I find is handy).
Final drive unit - well, your choice, but I've got a spare one of these in my garage too - takes up less space than the gearbox.
some sundy items like turn signals, body parts if you foresee a need (battery covers can be bought new still, but are $50 apiece!).
Given enough time, I can rationalize keeping most any BMW parts - but I've also got a big shed/small barn!
