Well if you really want to, I guess you can...
I would save the money, and make it fast instead. If you want an Si, just buy one
However, back to your question...
You would need a 99-00 front/rear bumber, side skirts, half red/clear tails, 99-00 head lights, painted door molding, rear disc conversion, and...Ummm I think thats it.
Did I forget anything????