Late model S13 and S14 SR used the "fat" O2 sensor, just find a replacement. For my S14 SR I just ordered up a Bosch replacement for a 97 240sx and it was exactly the same as the SR one. The wire from the sensor to the connector is only about 12" long on the S14 (part # 13420), so if you need a longer wire, you can get one for a 96 300ZX (part # 13091), those leads are 22" long.
Costs about $35 from rockauto. Can also get from local autoparts store, they're not "SR20/JDM" specific parts.
S14 Bosch replacement on top, stock on bottom:
