Few differences in S1 and S2 . few examples below , not all engine related but you may want to know


S1 has no ABS
S1 more basic loom , Does not run a temp sensor from the water supply , think its near the aux air device under the left hand side of the inlet manifold. Does not run a Knock sensor as on the s2
S2 can run on a S1 but not the other way round
S1 Has different gear box mountings , same as ECU . can run a S1 box on a S2 but not the other way round .
As per other thread the inlet manifold is slightly different .
Smaller 200mm clutch but dont quote me ,
Uses a lesspowerfull fuel pump , ( from the factory but probably replaced anyway )
S1 fuel accumulator is on the rear pasenger side near the wish bone and on the S2 its in the engine bay

Few others but thats all i can think of relevance