If you're looking for quick bolt-on power, get a turbo. Not a lot of bolt-on stuff for S2's along with motor parts. The S2 is right at nearly 11:1 compression ratio, so you can't boost too high with factory motor.

There is an australian that boosted an S2 cab, he produced 435rwhp before his rings gave out. He used a K27 @ 9psi. The car was full boost @ 2500rpm, lol.

Big power can be made with a hybrid 8valve 3.0 turbo, but be ready to spend thousands, even if you do your own work.

Or you can just buy a turbo. There are so many companies that have 300+hp kits for the turbo it's ridiculous.