The series 2 RST car had more sensors and a different ignition offset in
the software. (2 degrees different if I remember correctly).

If you use a S2 ecu on an S1 then the sensors that are need wont
be there and you will get unpredictable results.
This also depends on the chip fitted in the ecu too.
The extra sensors are a temperature sensor an the knock sensor.

You could retro fit the extra S2 sensors (with or without knock) then use
the newer ecu on the car and the dizzi adjusted correctly.


As for cosworth ecus, should be ok to swap as long as you understand the
other issues that need sorting apart from the knock sensor.
(TPS type, Lambda sensor etc...)