A JIS 27.0mm crown race will not seat securely on an ISO 26.4mm crown race seat, and an ISO 26.4mm crown race will not seat (and may in fact split if forced) on a JIS 27.0mm crown race seat.

But some headset manufacturers (e.g. Tange, Hatta, perhaps others) make the same model headset in both ISO and JIS dimensions, and sometimes the crown races can be purchased separately.

Another option if your frame/fork is milled to JIS dimensions, is to have it re-milled to ISO dimensions, and simply use an ISO headset.