Help! Garage door opener stopped working

Nope, I’m not stuck inside. I disengaged the door and closed it by hand this morning. I just need to replace the motor.

It was a quick diagnosis, since it’s an old unit without light screens and with no door connected. Just a quick couple of questions:

If I buy just any old replacement (or even same brand) chain drive motor, is it likely to be a plug and play replacement? Or am I likely to have to replace the existing chain, rail, etc., as if it were a new install? If the former I can do it; if the latter, then I’ll just call someone else to deal with it (not the money, but the time).

About that light screen… this is a grandfathered installation, not a new install, right? A replacement motor is just a replacement part? Anyone have a real cite for the law, because in the end, I will follow the federal law, even though I don’t want to (I prefer it as it is with a clutch).

Thanks!