New regulator assembly (with motor) would be my suggestion also because thats the best bet money and labor-wise. Replacement motors and regulator assemblies are hit and miss on quality / speed. Meaning you might have to buy and try a few times to get one that works as it should. I have one regulator thats slow / no-go like yours and it has worked exactly the same for five years. Think about it though, car window motors are not designed to run for 10-15 seconds of up and down. Normal usage is just one trip up and down within a minute or so, at least in my mind. Next time you have the glass disconnected, run it up and down by hand to be sure theres no binding in the tracks. Its not unusual to find the front track pinched at the top if a mirror has ever been damaged on that door.
