A good pre-electronic BMW airhead should last you a lifetime, especially if you casually spend an hour or two a month on ebay stockpiling spares. I've had my '86 R65 since new and I expect it will easily outlast me.

Ub
86 R65
05 K12S

PS: I have a '64 Corvette and there is not a single part on it that I cannot buy brand new at very reasonable prices. I had a good chuckle recently when I needed a new wheel cylinder - it set me back $9.50 at the local chain auto store, and it was in stock.