2001 Sebring Coupe engine swap check engine sign stays on
 

Hello, everyone.
Almost a year ago, my timing belt broke while I was driving home after work.
I had no choice but to get the car towed to the nearby auto shop, and I ended up changing timing belt, water pump, all the necessary work that needed to be done along with the engine swapping. (the work has been done by two different mechanic)

Anyway, when the mechanic swapped the engine he said that he didn't replace the computer because the new engine came from the same car. (I think he's talking about ECU/ECM, but you can correct me on that)
By the way, my car is 2001 Sebring coupe automatic 3.0L LX model, and the engine came from 2002 Sebring coupe 3.0L LXI model. (not sure if this one is automatic or manual)

So, now the check engine sign has been staying on since the repair, and I can't even get hold of the mechanic who swapped the engine now. (he now lives about two hours from me, and apparently changed the contact number also)

The car runs a little rough, but it feels pretty close to normal running condition. It's basically the check engine sign that concerns me.

I also took the car to the shop that I usually go to, but the owner didn't even bother to diagnose the car after I told him about the engine swapped.
(he told me to just sell the car as is, because it's not worth the repair cost especially after what I've already spent on the car)

If any of you who knows about the possible solution to this issue, or actually gone through similar experience, I would appreciate any advise.
Thank you.

all the information in the world but no code to what is making the light come on would be the first step