Anti theft light

Does it still turn over, just not start? If the anti-theft light is flashing with the key in the ignition and it does not start the first thing to try is a different key that has worked in it before. After that the problem is related to the actual system and you would need a scan tool that can access the security system to see which part is not working. There are pieces of the system in multiple modules as well as the key and the lock cylinder. The proper scan tool would show what is wrong.

You can also try a reset but it may do nothing if the issue is a hardware failure.
https://www.2carpros.com/articles/how-to-reset-a-security-system

That will require a bit more work. You would probably want to replace the key cylinder and have it repinned to match your old keys. An automotive locksmith can do that and would be much cheaper than a dealer. That would mean the current keys would work and no reprogramming should be needed and the keys would fit the other locks as well.

Isuzu part number 8973703220