JLC restoration question

So, the other day I ran into a vintage JLC for sale. The watch was not working and was pretty beat up, so it would have had to be restored for sure. Having no experience with vintage watches, I restrained myself and didn't buy it. However, seeing the watch got me thinking about what happens when you send a JLC to be restored (or any other comparable brand, for that matter). If the movement is no longer manufactured (which is almost surely the case) and no spare parts are available, does the company simply say they can't restore it? Or do they replace the entire movement with a newer one? Or do they manufacture the required pieces for the restoration?

Thanks for the info!