if you want to use the specialized base bars & cockpit, you need the specialized stem, as the specialized base bar does not have a round bar clamp.
if you are hell-bent on using a different stem, the steerer is 1" (not 1 1/8"). sure, you can use a shim, but most stems these days have stack heights (40+mm) that are greater than the exposed steerer on the shiv TT (~28mm). also, most stems place their bolts toward the back, and because the shiv TT is designed with aerodynamics in mind the gap behind steerer and top tube has been minimized. you might find the bolts interfering with rotation of the bars. additionally, you mess with the incredible aesthetics of the shiv TT. :) (this bike, in my opinion, is the best-/meanest-looking TT/Tri bike i've ever seen in the shiv TT incarnation.)
can you do it? maybe...if you're very motivated.
running most stems (other than something custom) would work against the very aerodynamic benefit that one hopes to achieve by racing this frame.