Vintage repair - RSC can't do it

[greeny]

Hello -
I have a 50-yr old Datejust that has been given to me and was in my family since new. Serial number indicates it was made in 1963. Was serviced a couple times about ~15 years ago by an AD; never serviced prior to that. It winds but won't stay running and runs pretty fast when running.

Recently sent to RSC by an AD and the RSC is sending back, stating they cannot service or repair as they no longer have the parts. I don't have the specifics (which parts were needed). AD said they've been getting this response from RSC more frequently recently when sending in vintage Rolex's.

AD said they've used a local watchmaker in these cases successfully and that they repair w/aftermarket parts. I'm not so much concerned about that, as that sounds like the only option - if Rolex no longer is making these parts available, then an authorized repair person (outside of the RSC) isn't going to be able to get them either and if they have used authentic parts laying around, they'd be just that - used and probably not trustworthy for a long-term repair. AD said they have to sometimes return them to watchmaker in a year or so (for adjustments maybe? we didn't get into details yet) but that it's much less costly than RSC, so money-wise, you've still spent less than had it been serviced by the RSC. Since this has been happening more frequently, AD was going to do some research and see if they thought any better options existed. To get one thing out of the way - AD is 110% trustworthy.

My question is this - in general how reliable are these aftermarket overhauls and repairs when parts are needed? Any other words of wisdom or recommendations?

Thank you!