You could get some material brazed onto the crown seat then re-cut it to JIS standards.

I've never heard of problems mixing a 26.4 crown race with JIS parts but I've never done it myself. If you could find 2 headsets of the same style for the 2 different specs (I guess you'd have to if you wanted to try this) you could just compare the crown races to see if they differ in any way besides the ID diameter. Sounds like a PITA though.

Or you could punch some notches into the crown race seat to see if you can tighten up the fit. If it doesn't work you wouldn't be out much since if you either braze on material or cut it down to 26.4, the notches wouldn't matter anymore.