Kudos to you for adding to your own post!
My first answer would have been "no one ever has" and Azusa has seriously dropped the ball on not having a live axle version of the Astro wheel. OR have they? If they had it, was it discontinued?
Ideally, any 1 inch bore hub, like for industrial uses could be used. Plain would be preferable so you could drill it to match the 5 bolt pattern on the Astro wheels.
Use standard 1 inch axle bearings. Nothing fancy needed.
Sorry you didn't get answers but this is a small go kart subforum on a site dedicated exclusively to minibikes. The go kart section gets less traffic*