you could also swap in a older CA18det
I would recomend staying with your KAde engine though, its got some good potental to make some power. some examples are, more displacement, better head design. some things that might get in the way of high HP are, not as bilt bottom end, long stroke (witch is good and bad), and smaller after maket.
SR is also a good engine, there is the red top witch was in the 89-94 silvia, and 91-94 180sx. then there is the newer black top with low port injection and vvt on the 95-98 silvia. then the not really updated black top (same engine as the red top w/ High port, but with a black valve cover) in the 95-98 180sx. then you have the 99-02 silvia black top, witch is about the same as the 95-98 silvia engine, but with bigger injectors, and turbo.
Then there is the CA18det engine witch was in the 180sx from 89-91.
here is a good wed site with more info on the SRs
www.srswap.com