Should I:
1) try to find a 26.4 crown that looks like the one on my old headset? How close must the crown be to the old one? The headset itself is a very simple steel job, upper and lower bearings are identical, 16 bearings per cage.
2) Buy a JIS headset from a company that makes indentical JIS and ISO headsets, try to press-fit the cups into the headtube (if they go in easily, it should mean that the headtube is 30.2 and so I can just go and buy the ISO headset of my choice). If they don't go in easily, then the headtube is 30.0mm and I need to buy a 26.4 crown from a matching ISO headset.
Just get a new ISO headset with the appropriate stack height (you haven't trimmed the fork yet, have you?), mill the head tube to ISO dimensions and install the new headset.