Replacement Rear Axle Sizes on Vintage Bike
If the cones are pitted, the bearing surfaces inside the hub shell may well also be damaged. As the rim is not particularly valuable, your local bike shop may be able to sell you a complete replacement rear wheel to suit the screw-on 6-speed block you have.
If you really wanted to only replace the axle, by 1990 I would expect a non-vintage hub to be using metric dimensions, so it's very likely it uses the most common size.