Its a lot more than just an engine you have to change.

ALL the engine management system--computers, sensors ,wiring harnesses, exhaust system, air pump, etc, etc. In addition, it is a violation of Federal law to install anything that might change emissions (even the gas cap) unless it was tested and certified in that configuration/installation type as conforming to the EPA requirements.

There is no rational reason for your 4.2 engine to be 'tired'. Have the timing chains checked for wear and get a good tune-up. A partially plugged cat convertor can seriously reduce engine performance.