As others have noted, the cones and bearing balls on that hub are pitted. New bearing balls and thick grease will help, but the cones will only get worse over time. There's a good chance that spalling from them will eventually trash the cups.

A German vendor on eBay has a NOS hub that might be a slightly (1990) later version of your hub. If you like the bike and want to keep it fairly "original", that might be a good "hedge" for the future - or at a minimum might be a source of new cones (no guarantee they're the same, though). Not cheap with shipping (just north of $31 with tax), but isn't that expensive for an out-of-production NOS hub either. Don't really know of any other good source for new Sachs-Maillard cones. Here's the link:

https://www.ebay.com/itm/255659503902

A couple of other possibilities: first: post your cone and hub photos in the "ISO and for trade thread part 5" in the Sticky section at the top of this sub-forum and indicate you're looking for a replacement set of cones. Many BF members posting here are incredibly helpful, and have amazing caches of vintage parts. Someone just might have a set of used but good cones compatible with your hub they can spare.

Second: take the cones and axle to an older LBS and see if they have something in better shape that might fit. (Take the axle to confirm cone threading is compatible.) FWIW: if I recall correctly, on some similar designs (Joytech and/or Sovus) the metal "seal ring" on the cone can be carefully pressed off the cone and moved to another bare cone with similar dimensions. I'm not familiar with the Sachs-Maillard hub, so that may or may not be the case for yours. But if it is, it will give you (and your LBS) greater flexibility in finding compatible replacement cones.

Best of luck, and enjoy the bike.