You have a standard ISO crown, nominally 26.4, but actually 26.45 to create interference. Unless you're in Japan where JIS (27.0) headsets are more common, you should have no problem finding the correct 1" headset. If you're totally unable to fine one, PM me as I have a bunch of new old stock 1" threaded headsets, and can take care of this for you.
BTW- take a moment to also measure the ID of the headtube, which should be 30.2mm or a bit under. Measure it in 2 directions in case it's slightly oval and average both for the true size.
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