Nothing wrong with spare parts. I usually have a couple air box base or plenum gaskets laying around. I tried reusing one once. Won't do it again. Not that you cant but, for a couple bucks, why risk a leak?
It's a shame there is no certainty of parts availability as these bikes age. I have no suggestions as to which parts you should squirrel away. Your riding style and paying attention to what wears out quickly is about all I can say. Hopefully, parts will outlast me and my kids will inherit a rare antique that people will pay ludicrous amounts of money for someday.