Since they are NOT the same as the later 4 ball caps which are not all that hard to find, the only answer I know of is to machine the nasty part of the cap off, make a new cap and solder it on to the old base. Not as hard as one might think but like all of this -- time consuming. They all have to be exactly the same height, same taper, same etc. These are not totally done, they still need a little more polishing but this is the difference between soaking them in carb cleaner, blowing them out and wiping them off with an oily rag and restoration. Restoration takes time and patience.
If you have a five ball and it doesn't run well, take the caps out and look at them. Shallow cup caps from a 4 ball will not allow the balls to raise far enough to flow enough air to run well under load. It will start and idle but the 4 ball balls are larger and heavier and the caps are different.
