Can you not grip the pinion in a vice and turn the nut that way..? or large spanner on the pinion or adjustable spanner? Th teeth mean it has flats on it .... make sure you remove any locking device on the nut first
While you have it off replace the brushes.While the armature is out put it in a vice(wrap it good.
When you re-install a bit of grease on the worm gear.
