I'm going swap out the headset on an old Bianchi frame, beacuse the current headset is brinnelled and I want to learn about headsets. A friend has all the tools (I think) I need. I know I'm looking for 1" threaded headset.

Since the bottom bracket is Italian threaded, is it safe to assume that the steerer is, too? Sheldon says that ISO and Italian headsets are interchangeable, so long as you don't go swapping them back and forth all the time. If this frame has had the same headset for 20+ years, do I need to worry about whether the threading is Italian or ISO? I'd certainly prefer to use ISO (easier to find).

And what about crown race diameter? It looks like most headsets are available with 26.4mm and 27mm crown race diameters. Again, Sheldon says that ISO headsets have a 26.4mm crown race diameter, and Italian headsets have either a 26.5mm or a 27mm.

So my questions are:

Is there a way to determine whether I have an Italian or ISO threaded steerer?
If it is Italian, can I safely order an ISO threaded headset regardless?
How do I determine what crown race diameter I need (I don't have calipers)? Or does it matter?

If it helps, it's a mid-80s Bianchi Rekord with an Ofmega headset, Italian threaded BB, Made in Italy sticker. It's gaspipe, though.

EDIT: I didn't quite realize how difficult Italian headsets were to find. Forget that. Now I just need to know what crown race diameter I need. Any guesses before I take it to an LBS this weekend?

Before you replace the headset, you could give this a try: rotate fork crown and lower cup from their prior orientations. I try about 80 degrees clockwise for one, counterclockwise for the other. Then use loose bearings in the lower race. If the upper race is also brinnelled, you could do the same here. I have done this with a Record headset, it worked fine.

Yeah, I think looking for an Italian headset out of the question. But I do need to figure out the crown race diameter.

since you're asking for guesses, here's mine:
your ofmega HS would have had a 26.4 crownrace and ISO threading, or close enough that you can go by that and go buy that. If you want to know for sure, then remove your HS and let your LBS match the old one with new. For my money: I'd buy a Tange Levin with 26.4 race and be 90% sure it would work.

This is very old school and a practical way to extend the life of any brinnelled headset as long is it isn't pitted in multiple places.