Ok first off this is not my car and i have not even see it yet. It belongs to my Dad's co worker. the motor is a 3.4 and the timing belt broke. I plan on taking the head off to see if anything got destroyed when the belt broke. However lets assume the motor is destroyed does a 3.1 bolt in if so i would replace the ecu with a 3.1 one. Just looking for some advice before i start this project thanks

As far as the 3.4 engine goes, that year model is an interference engine. Which means you'll likely find bent valves or cracked pistons. As far as the 3.4-3.1 swap, both engines are virtually identical, even down to component/sensor locations. The transmission will accept either engine, but the PCM will need swapped, but i'm pretty sure the wiring is identical.