B18C5 would be my recommendation; anything larger starts to upset the already delicate weight balance. Since you said this is going be to your track car, I'm guessing you would prefer balance over straight-line speed.

Also the S2000 is RWD; civic is FF, so you already have mad problems there.

You stated you want 250 NA hp, is that 250 crank or 250 to the WHEELS? The latta will be nearly impossible (unless you've got mad bling)...but either way you're going to have to trailer the civic...

Shingo