You'll need to measure the steerer tube and headset and subtract the difference to find the stack height. Pretty much any 1 inch threaded headset that matches that stack height will work.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"