They are cup and cone bearings and easy to turn up on a half decent lathe from an appropriate case hardening steel. Usually the cones wear out at three times the rate of the cup so often you only need to make them. As they revolve slowly (600 rpm max) they do not need to have a fancy ground finish, a good turned finish is more than adequate. The originals only ever had a turned finish.
Modifying the hubs to take modern ball races is just as much drama which is why I never bother. Nor is the flimsy hub that amenable to the idea. I stick with cup and cone.
The obvious place to enquire about materials, case hardening, an appropriate machine shop, etc, is your local model engineering club. You know, those blokes who run miniature steam trains in the local park. It is a meat and drink problem for them and they will point you in the right direction.
Cheers,