There you go telling the whole world that you put a non-certified part on your airplane!! A while back there was a discussion on Pilots of America about the ridiculous cost of a small part. I suggested that I could make it in about 10 minutes on my lathe and no one would ever know the difference. The whole forum blew up, you would have thought I publicly suggested not filing a flight plan!
It's a homebuilt--(almost) everything's non-certified

This alternator is for a 1995 Dodge van; cost me $130 from Summit and has all the power I need vs. the $500+ "airplane" model that puts out less power. I can pick up a replacement from any auto parts store if needed.
I need a little more power as I'm using automotive-style electronic fuel injection (
http://sdsefi.com/aircraft.html) which runs about 10 amps between fuel pump, ECU, and coils; the amenities like heated seats (to keep the wife happy) draw a good bit of power too.
I think the last time I filed a flight plan was almost 19 years ago, on my long solo cross-country
What is CRES?
Internet thinks it might be Corrosion REsistant Steel (?) SS303 or SS304 (?)
Yeah, your interpretation is correct, it's stainless. 416 round stock from McMaster.
As suggested, I'd just press them when required at the time of change.
Maybe better would be making the tool you need (pilot driver, etc) right now and carefully labelling it. That would save a lot more time later in my opinion.
Yeah, one way or another I'm going to make the tool now and store it. My big concern is with a failure "on the road"; as noted I could probably get a replacement alternator anywhere, and if I'm close to home I can fly back on the secondary alternator, but if I'm way out I'll need to replace it in the field. I'll need the bushings to make it fit, and I'd really like to have a spare set in my "flyaway kit" (the bag of tools and things like jack adapters, spare inner tube, "get home" alternator belt, etc) along with the bushing press/puller. That way I maybe won't have to press the old ones out in the field, and if I booger one up I'll have a spare. Can't feasibly lug my lathe around with me, after all, even if it is a HF benchtop model.
Don't think we need to debate that.
How long will these new bushings last and how well will you store this new information. Is it truly worth pulling them out to get information for a part you needed and made without the information in the first place.
The ones I made should last years; honestly they will probably be lifetime (of the alternator) parts. It's if/when the alternator dies on the road that I'm concerned.
The parts would be stored in my flyaway kit; I'd keep the dimensions stored electronically with all my other documentation (I have the build drawings, wiring diagrams, parts lists, and so on stored on Google Drive and my phone/tablet), as well as maybe noting them in paint pen as described above.