My LSMG told me a couple times to take the belt off and run the motor at full speed till it cleans itself out. You'll hear it change pitch. I tried it and it worked. I'd at least try that before taking it apart.
Oh, you can check the brushes by unscrewing the two Bakelite brush caps and pulling out on the brush springs.
If they are worn you can buy new ones at Sew-Classic.
Joe