If there were any security system issues, the "Red Dot" would have been illuminated or blinking with the key in the ignition and in the "Run" position. If the lamp turns on then goes out, the security system is operating properly and that is NOT the cause of the no start issue.
What I suspect has happened is during the time you had the doors open and doing the replacement of the motor, that the battery which is most likely weak since it is as old as the car has run down and doesn't have enough current to operate the starter. There may be enough for the lights but not for the starter and that is why I suspect that you need to replace the battery....
Keep me posted and let me know if you have any additional questions...
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