Performance wise you'll need a SJ filter/Lennies Induct and a Power Commander.
Wheels that'll be either PVM or Dymag particularly the front.
At the very least you'll need an aftermarket shock on the back the standard stuff is Boat anchor quality.
There's are plenty of ways of making an S lighter the problem comes if you don't want to pay.
If your really serious I'd look at Gus's bike on Boxertrix that will determine if your just tinkering or not as the case maybe
Oh and if your really lucky look for a set of aftermarket brakes.