[8695Beaters]

Actually the reason parts for the S2K are so expensive is not because of legality (well that's part of it), but mostly because finding any sort of gain takes a lot of work. Mugen would be the place to start for you. The parts are very expensive, but well worth the price of admission. The first upgrade you should make is to Ferrara valve springs and titanium retainers and keepers. The S2K has notoriously weak valve retainers which will wear through over time. Better to get them taken care of now if you plan on racing the car at all. Better valve springs will also prevent valve float which may happen if you spend a lot of time in the red zone of the tach.

Just do your research before buying anything. And realize that most parts will only gain a few hp, because that is all Honda left on the table. However, the S2K can be massaged in the chassis department quite a bit, so that would be a good place to invest money and time.

You are in great luck as this month's Sport Compact Car has a shootout between 9 different S2Ks. They are all mild street machines, exactly what you are looking for. See what parts they run. It will give you some ideas.