I do think caged bearings are relatively standardized, with a few different common sizes, and should be able to be replaced. It is noted that lower headset races & perhaps cups tend to wear by developing little washboards, or "indexing". Replacing caged bearings with loose bearings can help alleviate some of the issues of the indexed headset.

There was a question earlier about headset caged bearing orientation, and I noticed that there were two different types of bearing cages.



The cages seem to be mounted in the opposite direction, and I'm not sure if they are interchangeable. But, it is always easiest to bring samples in with you when shopping.