i would give it a full tune up:

fuel filter, airfilter, sparkplugs maybe wires, cap/rotor, maybe the o2 sensor, but i would make that the last thing, becuase of cost.

the zc is only 130hp compared to the stock si which is 110 i think?? the exhaust manifold is the si manifold cuz the zc wouldnt bolt up to my stock exhaust. either way they are both 1 wire o2 sensors. fuel filter was changed at time of swap. new spark plugs aswell. i hate those loud exhaust so i stuck with the stock one which leaks abit but nothin major.

Please please please take the picture out of your signature.

I would have an exaust back presure test done to make sure that your cat isn't cloged, and that your exsuast doesn't have to much back presure, when they do that they have to pull the o2 sensor, and a mecanic that has been working at the shop for more then a week should be able to notice right away if your o2 sensor is black and sooty (running rich... bad o2 sensor), you can also check your spark plugs for simaler sootyness.
what type of air intake do you have?
the zc is 1.6L right? any idea what RPM you are running at in 5th gear at 60mph?