S1 has 2 temp sensors too, one for each ECU.

You can't run an S2 ecu on an S1 without a knock sensor. It will still retard the ignition. You can compenstate to a degree with the dizzy, but it will never be right. This goes for shorting the knock sensor wires too. Sending 5v straight back to the knock sensor input will have a an effect on the ignition timing too.