Let me turn this thread around somewhat: Which mechanical parts would you suggest finding/keeping that are obselete or in very short supply for our classic SERGs?
None of the mechanical stuff is a HUGE supply issue; yet. Though some of the it is getting expensive. Even the M&M throttle bodies still have an aftermarket. A stupidly expensive aftermarket but it's there. Stock M&M throttle bodies come and go on eBay. It's not a consistent market but it's not depleted. ECMs are the same way.
The old harnesses are going obsolete from Harley. So watching for some of that isn't a bid idea. The engine harness is nice to have a spare of. I've got one very good one and a couple that are a little beat up. They are just for connectors as parts pieces should I ever need to make a repair rather than replace.
The old instruments are becoming a concern. Harley has moved well past them. Again there is an aftermarket but the silver SERG specific instruments were never a high use item for the MoCo anyway. Even more so now for the pre-04 vintage in question.
As Mother Harley integrates more and more in to the ECM eventually things like TSMs or TSSMs will become harder to get. But that's still a long way off. Someone told me a month ago they couldn't get a lean angle sensor, that it was back ordered for a month. Never verified but sent the guy an old used spare I had on the shelf.
TSSM and bank sensor go away on the touring models if you've added the alarm system; so some of that becomes moot depending on the upgrades done. Of course can't add the alarm system anymore because the supplemental harness needed to do it has been obsolete for a couple years.
Suppose if I were planning for long term use I'd stash Marelli fuel injectors, an engine harness, the fuel and engine sensors and the gauges, and an ECM as I could find them cheap on eBay. Some day any of that might bite us in the ass for availability. The stuff does still show up though. So no reason not to be patient and get cheap scores on eBay or elsewhere.
That's the mechanical side of the question. The bike specific painted parts are an individual choice. The fenders are solid colors and easily matched. Side covers, tank and fairing less so. They'd also be frighteningly expensive to get painted so (even though you could) if I found cheap scores I would. The rest of the mechanical stuff (suspension, drive, etc) was used on so many bikes for so many years it doesn't worry me.