If the motor has standard 2-wire connections (not 4 wire), it should spin if it was wired correctly. As there is internal speed control circuitry present, it can not be wired in reverse.
Replacement motors are available. They might not look 100% the same and generic replacements are often slightly smaller, but the mounts are the same and they should function the same too. Just need to figure out if it is CW or CCW rotation. You might have to spin the rotating assembly to determine which direction makes it work properly (look at capstan needle to determine if it is spinning in the proper direction). The needle should rotate in the direction that would cause the tape to feed to the take up reel. Also replacement motors are almost always 2400rpm, which fortunately is what most sets require. If you need one that is different, then they will be hard(er) to find.
I have (new) motors available (12v, 2400rpm) in both CW and CCW rotation. If you are in USA, I can send you one and the only thing I ask in return is that you make a donation to the Boomboxery website of $12 or more. Send me a PM if you are interested. I am merely a member and not an associate of Boomboxery.com, so no funds go to me, I'm merely doing my part to help out the website that makes all this possible.
. Below is my boombox and am looking for belts, head, pinch roller & motor.