As a general rule, I don't bother trying to mix headset parts, except for similar models from the same maker, which often share some parts. (it often can be done so it works, but you have issues with seals, excess clearance allowing water in, and/or fit differences enough to cause poor/unacceptable cosmetics)
Your best choice is to source a new JIS headset, because even if you recut the fork to accept a 26.4 ISO crown, you still have to deal with the fact the JIS and ISO heat tube dimensions are also different.
Or you can have the crown seat milled at a bike shop, but then you'll be struggling to mix JIS and ISO headset parts until you also ream the head tube to ISO.
On the bright side, both JIS forks and headtubes can be recut for ISO, so if you ever run out of JIS headsets you'll still have options.
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