There's plenty of vintage Italian headsets available on ebay. If you do not need cosmetically perfect they can be very affordable. Look for Campy Record from the 70's or Magistroni, which is what the Olmo would have came with (although branded as Olmo.)

If you are going to modify something, do NOT cut the fork crown. You can easily enlarge a 26.4 bearing race by chucking it in a lathe and using a die grinder and stone to open it up. Just go slow and measure a lot.