Yeah, the motor is probably old, and the Jeep is a '91, and there's a federal law saying you can't put in a replacement engine from any year older than the vehicle.
There's also a Federal law saying you can't remove or tamper with emissions equipment. I didn't see any mention of catalytic converters on the true dual exhaust, so I assume there are none.
All of which means that this rig is illegal and could not even be registered in about 75% of jurisdictions in the United States.