most of the suspension parts, strut tower braces, and stuff can be swapped on. Possibly the exhaust headers and underdrive pulleys will fit, however this has yet to be confirmed. In a NPM issue a few months back, someone covered the B13-B12 suspension swap. Strut tower braces fit, Nismodriver has one on his car i believe.
Engine parts swaps are limited by the fact that the B13 has MPFI system, whereas B12s were either TBI or carbed.