Lost key, ignition re-installation

Well, the easiest thing would have been to contact a locksmith and have them make you a new key and program it into the system while deleting the old lost keys.
The question now is which parts were removed. Just the lock cylinder or was the steering column taken apart to change out the entire ignition switch assembly? If it is only the ignition cylinder and key, installing it is simple, put the key in, turn it to the run position and then line it up in the ignition switch and push it in until it clicks. Once installed turn the key off. Now you need a scan tool and 2 keys that fit the ignition switch. It will not start until 2 keys have been programmed into the security system.
With the proper scan tool you can access the security system and follow the process below to program in the replacement keys.

1. Turn the ignition switch from the OFF position to the RUN position.
2. Using the diagnostic tool, follow the Security Access menu to obtain security access for the power-train control module (PCM).
3. From the diagnostic tool menu, select Ignition Key Code Erase.
4. Do not select any additional commands from this menu.
5. Turn the ignition switch to the "off"position and disconnect the diagnostic tool.
6. Insert the first encoded key into the ignition lock cylinder and turn the ignition switch to the "run" position for 3 seconds.
7. Insert the second encoded key into the ignition lock cylinder and turn the ignition switch to the "run" position for 3 seconds.
8. Remove the second encoded key from the ignition lock cylinder.
9. The vehicle should now start with both ignition keys.