A good start would be to take it apart and see how bad it is. The crown race (at the crown of the fork) is probably shot, but there's a good possibility you could find that separately if your cups and upper cone are OK. Ten bucks on amazon will get you an assortment of grade 25 balls in common bicycle sizes that fit everything except bottom brackets (cranks). Another 6 bucks gets you some 5/16" balls that fit cranks. If your retainers are OK, just pop new balls in. If not, the retainers with balls in them already may be available from someone here on the CABE or ebay, but you would have to take them out to see which ones they are.
That's the easy way.
If it is all truly shot you'll have to find some new stuff to fit. The critical dimensions are the inside diameter of the headtube, and the outer diameter of the steerer tube where the crown race goes. Measure those with a caliper.
If your headtube is oversize, Wald makes one headset with 3 little bumps on the outside of the cones that might work. If not, or if you want something better than Wald, you may need to make some shims. Every one of my recent projects has required making shims.
On the other hand if the headtube is standard (the same size as Chicago Schwinn and older BMX), you may have several choices for whole headsets that will go right in. You still have to pay attention to the size of the steerer tube to make sure the set you buy also has a crown race that fits the steerer tube. There are 2 common sizes in American bikes.