If memory serves, thinking the bearings are tapered roller bearings?
That makes it more difficult, but,,,,
You might be able to go to a bearing supplier, and find a set of bearings/races that fit the ZG bike neck / ZX stem bearings.
I've done that on projects over the years and was surprised to find bearings that did the trick.
Or (mebbe) you could install a ZG Stem into the ZX Tripple tree? (I think their pressed in)
Ride safe, Ted