Diagnostics

There may be more than one concern here. First of all, you need to get the vehicle to crank with the key. It may require a theft relearn for the cranking. You almost need a scan tool to verify after it's able to crank, that you have engine rpm present in the ECM. If you have no rpms shown, it is not receiving an rpm signal from the crank sensor. I don't believe you will lose spark and fuel injector pulse if a theft fault is present, but maybe. You may find that after a theft relearn is performed, it may start and run. Looking at the system operation, it looks like a theft system problem will only result in no injector pulse. Check for blown fuses, and also all connections.

Was just the engine replaced? It sounds like a theft issue that may be corrected with a relearn, the problem is, access to gm programming is needed to accomplish this. What does the security icon do at key on, does it stay illuminated? My guess is that there would also be a code stored, possibly in the powertrain controller. Do you have access to a code reader? If so I would check for powertrain codes. If there are no powertrain dtc's, it could have codes stored in theft deterrent, but a code reader would probably not offer them since most all of them do powertrain only.