The headstock overlay is made from Celluloid which shrinks over time. So there is not much you can do. You would have to remove all tuners and you can glue it and clamp it and it would last for some years again. There is celluloid glue for accordions for instance that can be used. But if done wrongly the glue might leak out during clamping, penetrate the good finish of the headstock and destroy it where the glue comes in contact. So it could look worse afterwards... So yes, glueing can be done (but it needs to be clamped for some days as warped celluloid is very tough to bring into a flat shape again).
The below two pictures are from our member fixit, who is a luthier in Florida (Jacobs Custom Guitars) and worked in the Westerly factory. That was a 1967 Starfire IV he restored.
Some luthiers (Dave Petillo in the below picture) or even the former Guild Nashville custom shop repaired such extremely warped celluloid veneers on '60s headstocks by using a new black ebony wood veneer but you need a new Guild inlay which is not available unless somebody custom makes it. If done correctly the results look good afterwards and Guilds are not '59 Lea Pauls value wise...
Ralf
