The S1 wing has a round wheel arch and the others have a squared arch.
Yes you can cut a s2 or 3 wing down to fit and plenty of people have done it but your car wont look like a series one anymorehttp://www.slroc2.co.uk/forum/Smileys/default/sad.gif
Finding Series one wings is hard. I am surprised you cant buy plastic ones.
Pete.