Stuff breaks. That's a fact of life. Everything ever made by the hand of Man is, on any kind of significant timescale, temporary.
There are ancient cars out there, still moving around under their own power. There are ancient refrigerators, ancient houses, ancient books that are still legible. But they must all inevitably cease to be eventually.
The good news is that we can shove that universal deadline back. When things are made in such a way that they can be repaired, you can keep things moving.
This can go to extremes. You've probably heard the joke/philosophical gambit that goes, "Yeah, grandpa had a great axe for decades. Replaced the handle only three times, and the head only once. That was a great axe." You see the complication here: an axe only has two parts: handle and head. To replace a handle is to repair the axe, but it's more than half of the entire tool. To replace the head is to replace the entire tool's business end. Without that the handle is only a stick.
So if you replace anything on a car, your car is no longer completely original. It may still be factory, however: if the parts are still produced by the original manufacturer, then you can safely say the whole thing is still stock.
I found a new stereo for my Subaru at a junkyard. The stereo was marked with the Subaru name and was original equipment included in a Legacy of about the same age as my car. I took it out of the junked Legacy and installed it in my car, where it fit perfectly, even using the original wiring harness connections. But that stereo was never offered in my car. So it's factory, but not stock.
How many parts do you replace before the car doesn't fit the bill of being what it was? You can literally buy all the parts to build an entire 1964.5 Ford Mustang from scratch. If you're really into the whole build-it-yourself idea, you could build the engine a part at a time, starting from a bare engine block. You can also buy an entire frame to build a classic appearing pickup truck, a 1950s Ford F1 or a comparable Chevy. You can make it as vintage or as modern under the skin as you want.
It isn't a Ford or a Chevy, of course. The frame built by niche manufacturers, the body panels built by hobby suppliers keeping the vintage truck market in new steel have nothing to do with the mighty corporations in Michigan.
So I need new tail lights for my Volvo. Volvo built the 240 for about 20 years, and they pretty much nailed down the design early on. The tail lights for my car are specified to fit a range of model years from about 1983 to 1993. I think the company that made these tail lights is in fact the original manufacturer, a supplier that provided parts for Volvo when the car was still in production. So strictly speaking, these may be original equipment, even though they aren't strictly factory as they were produced after the car's production run ended.
This is a thing for a lot of people. When parts aren't original, there are purists who suck all the fun out of having vintage equipment. I saw a beautiful Farmall Cub at an antique tractor show, one of the most nicely appointed examples I had ever seen. The guy had replaced every exposed fastener with the same size, but in stainless steel. Really dressed the machine up. There wasn't any chrome in evidence, not even on the exhaust pipe. But all that stainless really gleamed and of course there wasn't a hint of rust to be seen.
That didn't stop the occasional sour-faced grump from pooh-poohing the guy's effort. "Too bad he couldn't be bothered to keep it original."
"Too bad you couldn't say anything nice about it. He's put a lot of work in keeping an old piece of hardware in good running condition. Where's your tractor?" The guy looked at me like I was both insane and incredibly rude, and stomped away. I don't often talk back at people like that, but it just seemed like it needed to be said.
I need to build a three-point hitch for my Farmall. It doesn't have any hitch gear at all right now, and International's answer to the Ford-specific "Ferguson System" three-point was their own proprietary "Fast Hitch." But the incredibly logical and effective three-point, coupled with Ford's gigantic market presence, forced the other manufacturers to adopt the three point as standard equipment. I could find Fast Hitch implements, but it would be a long, hard slog and frankly I don't want to do that. I want to build something. I'll paint it red so at least it doesn't look too weird.
I had a point that I was working toward when I started, and I think I lost it. I'll try to pick it back up in the not-too-distant future.