If the motor spins OK without a tape inserted but not when there is one inserted use a Q-Tip lightly moistened with Isopropyl Alcohol 99%+ (available from electronics parts stores) to clean the pinch roller, capstan and playback head. Also check that the belt is clean as well and that it has not stretched.
Check if the cassette tape drive and take up wheel spin freely when the belt is removed just in case they're sticking for some reason.
If still no good it may be that the motor is not producing enough torque to turn the cassette.
You may have to check the motor drive circuit to see if enough voltage/current is being supplied to the motor.
Here's the service manual that may help with this.
If the belt has stretched and needs to be replaced, search online for 3-354-868-11 to find suppliers that suit you best.
The manual has a complete parts list so if any other parts needs replacing use the part number to find suppliers.
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