Is there a particular type of torc that you are interested in?
There’s quite a wide variety, using different methods of
construction - from plain barswith a round terminal, to twisted
strips, to complex woven chains.
Once you have decided on the type, things might not be as they first
appear. For any of the heavier solid types, you’d need to cast a bar
and forge it to shape, perhaps gouging out grooves which can then be
twisted to give a spiral. For the lighter types, you might hope to
use modern drawn wire, but this is probably innaccurate - the celts
may not have had access to draw plates, and so even the finer wires
would have to be hammered or swaged (although there are other ways
of “faking” round wire).
In some cases, the torcs might be completely cast - this would
certainly be simpler, but isn’t going to be relevant if you are
trying to produce flexible types.
There’s a good paper from Gold Bulletin, “Celtic Gold Torcs” by
Christiane Eluere, Volume 20, 1987. If you can’t find it online,
email me, and I’ll send you a scan of it. Another good source if
you’re interested in wire twisting is Herbert Maryon’s book
"Metalworking and Enamel", which has an excellent section with
60-odd different ways of twisting wires.
I’ve probably got a few things I can email you, but I’m not going to
dig them out unless you are genuinely interested, so don’t hesitate
to email me if you are, and I’ll see what I can do. Otherwise, an
aggresive google session should yield some good PDFs.