O2 Sensor for CA20E URGENT HELP PLS

Hi everyone

I recently bought a 1984 Silvia S12 or Gazelle with CA20E engine
but according to the previous owner they got it repaced with a newer one. Someone told me that a 1984 S12 doesnt have O2 sensor
so this engine must be 1986 +

now My problem is that I cant seem to find the wire that connects to the O2 Sensor, I've look in my engine bay for many times and no luck.

so I'm guessing the the previous owner did not upgrade the ECU that match the engine.

Can anyone help or give me advice on how to solve my dilemma

if I buy the matching ECU 1986+ would this be a straight swap to my old one or do I have to re-do all the wiring/harness?

thanks in advance


Cheers