Reddit mentions of Origin8 1" Threaded to 1-1/8" Threadless Quill Stem Adapter - Silver

You need a 1" quill stem or a threadless adapter like this:

http://www.amazon.com/gp/aw/d/B0037N6PEC?pc_redir=1412262710&robot_redir=1

The adapter lets you use modern bars if you do desire, as there are no quill stems for 31.8 bars.

Won't need a new fork or headset, just a new adapter, and a new stem.

Out of curiosity, what Centurion did you pick up? I have an old '86 Time Machine I've been thinking about converting.

Yeah, I don't see why not. A shim doesn't noticeably effect performance at all, especially on a commuter bike. Something like this will do the job perfectly as an adapter. Or, you can get a new quill. Either way, you'll spend less money by purchasing all that, than buying a new 1" threadless fork.

EDIT: 1" threadless carbon fiber fork.

You might want to think about one of these.

I've gone both ways on this, working from pretty much the same motivation as you with my intent, frankly, being in large part wanting the more modern look.

I put a new fork and a threadless headset into an old Ibis and it worked fine but cost me some money and didn't have any functional advantage over the adapter approach I took on a Giant Rincon of the same vintage. If I were doing it again I'd probably shell out the $20 for an adapter and the $15 for a 1 inch headset and I'd see if that got me where I wanted, because if it didn't I could always go back and do a 1 1/8 fork...

Hm, so since you have had one I'll just show you what I was looking at for a headset (if you don't mind).

Headset

Stem adapter

Stem

I used a stem adapter made my Origin 8! http://www.amazon.com/Origin8-Threaded-Threadless-Quill-Adapter/dp/B0037N6PEC