Help with immobiliser issue

[Gav_180sx]

I have a mk1 1.8 Mx5, car was running fine until it started cutting out after 2 seconds or so. Changed relays and fuel pump which didn’t fix anything. Got a fault code p1602 immobiliser pcm communication fault

Not sure at what point the communication is failing, if it’s the key, immobiliser or ecu. Is their a way of ruling some out?

If I can get ahold of an ecu, immobiliser and key from another mx would it fire up or is their anything else that talks to the ecu or immobiliser?