The S1 rear is significantly narrower I recall so modifying one is unlikely to work. It would be possible to create a bespoke one of course for the right investment.
Converting the s1 to side exits is non trivial as the sills are thinner (it was of course managed on the GTN) though you may run into issues with sharper turns on a narrow bore pipe restricting the exhaust. Factory used to supply parts, but the s1 conversion was well over 2 k plus vat plus fitting. The s2 etc were less than half the cost. These parts are probably also no longer available (they weren't exactly in demand at the time) so it may be the case of going to a specialist exhaust manufacturer - at which point with an S1 you are doing a non factory mod which may restrict your ability to sell when you want to move the car on.
On the diffuser, remember that its purely cosmetic, adds weight and drag and provides no performance benefit. Looks cool though. I had one on my s2 and s4...
