hi, it does indeed have an imobilizer, the CODE system, and that needs to be removed along with the ECU and keys. It is not a problem if the engine is going into another Alfa, as the mechanicals with be using the original ECU and key set, but if going into another car altogether, you need the complete set of electricals.
The code box is under the dashboard, open up the fuse box area, look up and look for a green ECU type unit, that is the CODE box. PITA to remove, but for security reasons it is meant to be. The ECU is under the passenger side carpet, peel back, remove the black plastic cover, undo the securing nuts and unplug. Might be an idea to retain the wiring loom also for the engine/ECU/CODE unit etc