Headset standards: EC29 vs EC30
It's unclear how it came to be a JIS (EC29) frame with threadless. That combination is odd. There are some different ways it might have been hacked together, or maybe some small number of bikes were made that way. It doesn't matter. Get the frame reamed to ISO/Campy/EC30 (all names for the same thing) and the crown milled if needed, and then you can install an obtainable new 1&quot; threadless headset on it.
Some manufacturers of basic OEM and repair type headsets use the same cup profile for everything. Taking advantage of that is one way you could in theory combine two headsets to make a good hack solution. But it would be way easier to just machine the frame.
Jamming an EC30 headset in should not be done under any circumstances. The frame will be at risk for cracking now or later.