No idea if AMC used the same as Packard but the broken armature is sure familiar so it is possible. The Studebaker part of the corporation did not use the same so I expect there were several suppliers a mfg could choose from.
Packard used Delco motors in 55-6. Round, approx 4" diameter with a yellow and black wire. Case is grounded with one wire used for each direction and power fed directly from the switch. New motors are not all that common today but do come up from time to time. More common is finding them used -- both can be fairly expensive though. Shortly after Packards demise Stude was having problems supplying the exact Packard motor so made a kit with a substitute. Those are also somewhat hard to come by today. Even if the motors are the same another variable may be the regulator mounting and the number of teeth on the output drive gear. I believe Packards gear has 9 teeth and has 3 screws thru the gear assy portion holding it to the regulator..
If the Delco motor sounds promising Kanter
https://www.kanter.com/packard/pac-556.html#4 advertises used is available in Gr 30.3861 and they also come up on ebay from time to time.
One of our posters had his motor and/or switch go out once too many times so finally resorted to changing out the entire setup. He used a modern universal street rod type power window assy. It took a bit of fabrication making a plate to mount the new assy and some relays to be able to use the original switch but from all reports has worked well.
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