For an SR swap it's going to be complicated, and its going to be expensive. Spend time on some sr20 specific forums if you want to learn more about it. SR info is kind of lacking here on hybridZ.
A simpler swap would be another L6 motor. Like they were saying, a well tuned motor will get good gas milage. Your driving habits will have a lot to do with the milage you get.
Part of the reason why i'm doing an SR swap is good milage, because of the 6speed available. I plan on doing a lot of highway driving. [up and down the 5]