If you buy a headset, there are a few measurements you'll need to know. It's a 1" threaded headset, but there were multiple 1" headset standards. Being a Japanese bike, it might be sized to Japanese Industrial Standard (JIS) instead of the ISO headset sizing you'd find on an Italian-built Bianchi.

If you have a caliper, you can measure to determine which you have:
* Is the fork's crown race seat 26.4 mm (ISO) or 27.0 mm (JIS)?
* Are the cups 30.0 mm (JIS) or 30.2 mm (ISO)?

...and to make things a little more confusing, there are at least a few bikes out there that actually have mixed headsets. The crown race might be ISO and the cups JIS, or vice versa.

Velo Orange sells both JIS and ISO sized headsets.