There have been several threads like this. The only reason why people actually responded the first time around was because the question was about an engine that would rev to 11000rpms!!!
Ok now for the "revised" question, You can get a sr20de, ka24de, ca18de, even a 13b rotary that will all make good power in N/A trim, and will make the Z a fun (lighter) car. All of those IMHO would only be worth it (compared to the stock engine or a fuel injected L6 N/A engine) if you turbocharged them, because the stock engine will usually exceed the output of those. (N/A of course)
Oh yeah, I guess you should take the 13b off that list if you want reliability.
-Oliver