Just found that Rock Auto has new Brass/Copper heater cores for both AC and non-AC equipped Fieros. Reasonably priced. I would stay away from the Spectra brands. The aluminum core they produce is undersized. I expect that a new copper/brass core would be the correct size.
If you need one, or just want to put one on the shelf for the possible future need, get 'em while they got 'em. I've always had good service from Standard Motor Products and that's the one I ordered.