Actually, at minimum, you need (aside from a threadless fork and stem) just a threadless cup or cone to fit your upper bearing, which if loose ball, is likely to be a pretty standard size; it's probably the fit outside the race you'd be looking at. Also of course the appropriate wedge ring for the cup or cone, as these vary all over the place and usually don't swap well.

Although most threaded road headsets, and a few MTB ones, use a threaded cup; this upside-down style is pretty rare in threadless AFAIK so you'd probably need the whole half.