Hi,
Obvious bit is that the dash panel is different. S3 is mounted in front of steering wheel, and top is padded. Back of S2 is easier to access.
Almost as obvious is that the windscreen hinges are different, so you will need screen from S3.
I have vague memories of the steering column mounting differently on the dash. May be interchangable, can't remember, I only ever dismantled one S3.
There may be more, but that should be enough to get you started.
Brand new, galvanised S2 bulkheads are available, I think can be built to your spec. Memory says about £600.
602