well, your best options are an sr20det, or turbo the stock KA. If you are hell bend on a turbo 6 then an rb (inline 6) is your best option. What are you going to do with the car? 1/4 mile, drift or road course?

FYI, it's pretty easy to get 350whp out of an sr.

http://forums.nicoclub.com/zerothread?id=74575 <-----thread about installing an rb into an s13