I think you have a complaint with whoever did the work on the cooling system first of all.
But if you don't want to get into that, then yes, replacing the head gasket is a lot easier than replacing the engine. You aren't allowed to install an older engine anyway (technically speaking) as it violates Federal emissions laws.
It would be good to know HOW the new mechanic arrived at his diagnosis however. Ask for data/proof of why he thinks it is a bad head gasket.
Also we have to consider WHY the Jeep overheated. It might be that the head gasket having blown is only the symptom, not the disease.
If you let the second mechanic work on the Jeep, then the first mechanic is pretty much off the hook if he did something wrong.