OK ignoring the chinese stuff - motor option looks favorite - it's not belts slipping and to give you an indication of how little torque it has sometimes you have to start the chuck spinning by hand to get it moving!!!!
It has however seen very little use in the 20 years and certainly if it runs for a while it then starts up straight away
Take the belt off and see if everything moves freely as the grease might have gone hard in the bearings. The motor capacitor might be tired out with age too.
