I hate to say it but that is exactly what can happen if you wait too long to do LIM gaskets when it is leaking. You were just unfortunate enough to replace them and then have the engine go out.
Same thing happened to a guy at the shop. A car came in, grand am with 3100, blown LIM gaskets. The guy had still been driving it. The mechanic took it apart, replaced the LIM gaskets, got it back together, got it running, called the owner to pick it up. Owner came later that afternoon, checked the work, left the car idling in the parking lot while he came in to pay the bill, when he got back out, car had stalled. We pulled it into the shop, no compression on 3,4,5,6 cylinders. Tear down we found a siezed cam bearing which caused the camshaft to split right in half. Boy was that guy mad when he found out he needed a new motor.
You did change the oil and filter after replacing the LIM gaskets and UIM manifold, right? What brand oil and filter did you go with? Maybe it dropped oil pressure.
Either way it doesn't matter. Whats done is done. You can try Morads for a used motor. I am sure they have what you need.
http://www.moradpartscompany.com/