BS !! Just search for rotors. That’s my opinion , but you may have already tried that.
Always can upgrade to 80’s vented disks with a little more searching and a little money- not much. Then you may have an easier time finding rotors in the future.
To me items like brakes , rotors , and general consumables I stock up what I can. I’ve already lived the 70’s ownership where you couldn’t get parts from the dealer after 20 years or maybe less. So you tend to stock up when the wall hits you.
Sounds like warped rotors to me and maybe now a sticky caliper or two. Find the rebuild kits while they are still out there.
Maybe place the items in the guys hands and instruct him to return the removed parts to you. Keep them on their toes.
Best of luck with it.
Since it’s done so much sitting maybe give the wheel bearings some thought depending on when they were done last. Might as well spend the money right and take care of each corner as best as you can. Bearing , dust seals , brake pads and rotors. Then don’t sweat it for another 100k which you’ll never do anyways now that it’s so classic.
All just my opinion. Enjoy !