As denverDon said: "it's pretty much up to the emission standard at where you live..."
if you're in CA I have some tips:
- the engine has to be equal or newer than your Jeep
- all the emission control parts need to be present such as: EGR solenoid, AIR diverter solenoid, Canister Purge solenoid, AIR pump, O2 sensor/s
- some aftermarket parts companies offer emission qualified parts if you need to know these part you can visit http://www.arb.ca.gov
and surf to the database where you can search for any legal aftermarket parts with EO number.
- once you done , make sure you have everything ready, take it to smog test see if you pass then schedule an appointment with the BAR for a visual inspection. Remember each time you go there is $50 fee apply no matter you pass or not. Be careful here because if you don't pass they will be on you tail untill you corrected it. It's only a visual inspection test so as long as you have all the emission equipments you're good to go.
it's all your decision to make whether or not take it to the legal standard. I haven't heard any Jeep owner being pull over because of not meeting emission standard, it's only happen to the import compact vehicles. Keep in mind that it costs a lot for those certified aftermarket parts. It add up to your swapping cost.
that's much I can think right now...if you need more info I'll think harder .....