I never thought they were, but having seen aftermarket headrests for sale that claim to fit many cars, is there a standard size for the fitments across the industry nowadays?
Another problem will be the seat - has it been designed to take the forces from a head rest/restraint? 40+ years ago I doubt it. (Although there were horrible aftermarket things that slotted onto the seat but I always wondered whether they would work.) Just change the seat.