i THINK the box will go in, but an S1 box wont go into an S2! iirc.

differences engine wise are basically nothing apart from water pump (not much of a prob) and knock sensor on the S2 engine. which isnt a prob if you use an S1 ecu as it doesnt have the facility to use the sensor. but if u want o use an S2 ECU u will need an S2 engine loom. i have an S2 engine running S1 ECU in my S1