Hi! First time poster, long time reader!

I have a frame/fork with an italian threaded 1" headset that I think I should replace (the lower race has some pitting and the exterior has a crack, and one of the o-ring seals is cracked).

I took the bike in to a shop to have a new headset pressed in, but they stated that they didn't have any Italian threaded headsets, and that the english / ISO headsets they have would not work.

I read the following on the sheldon brown site:

"Italian freewheel and headset threads are the same as British/ISO, except for the thread angle difference. They may be interchanged, but you should not go back-and-forth bewteen Italian and British/ISO headsets."

Does this mean that I can just put an english / ISO headset on this bike? Does do not "go back-and-forth" mean that once an english thread headset has been put on the bike, an Italian threaded one will no longer work, or vice versa?

Here's some more info: The headset currently on the bike is a shimano 105 which I'd guess is at least 10 to 15 years old. I have trouble putting on the top threaded race due to a "lip" on the threads of the fork. There is also no washer between the top race and locknut - is this normal for this brand / type of headset? Is this even an Italian headset currently on the bike?

Any advice / info would be greatly appreciated!