Replacing the motor isn't easy. It's just as difficult to pull the whole regulator out with the motor as it is to separate the two still inside the door. If it's rolling up half way easily then your problem is more likely the regulator.
Look at the wiring diagram and try jumping the motor with a 12V source for both up and down. Then try it with someone helping move the glass and to see if they can feel any notchiness to the movement. If they push/pull too hard is may hide the trouble area you're looking for. I'll found that the nylon rollers on the regulator often either flatten out or break off at this age, then the moving armature hangs up on the edge of the track anywhere that the lip has begun getting chewed up.
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