It's not legal to swap an SR in many states, but some are more lax than others. The same thing applies to turboing a KA. The only actual legal swaps are motors that were released for sale in the U.S. with the same drivetrain configuration and that are newer than the car that it's going into.
That pretty much narrows your choices down to the VQ35, VH45 and LSx. But you can't just swap 'em and and run 'em on the street. Gotta make sure they're emissions compliant.
But if I were you, I'd just find a hook-up man who'll pass your car regardless, keep the KA24E and wait until Martins_240sx finishes and starts selling his Twin Charged kits for the Sohc...
