Also, as Jerry mentioned you could just replace the commutator brushes and clean everything up. There are rebuild kits available. Additionally, most of these older starters (and many of the newer) ones have "oilite" (spelling) bushings that both ends of the armature shaft slide into and rotate on (no bearings). These bushings utilize a form of dry film lubricant integrated into the bushing and that's why they don't usually have any lubrication requirement. If the bushings wear (as all things do) eventually there can/will be play on the armature shaft and that can lead to the armature making contact and rubbing on the magnets. That's one possibility. It is unlikely that a rebuild kit comes with new bushings as they are a pressed/integrated item into the commutator cap and the end cap on the other end. The other guys on the forum can confirm this, but I think you can order a rebuilt/refurbished starter (might require a core) at a reduced price. Another possibility is given that your starter sounds like it has overheated/is getting burned out, you are better off replacing it and eliminating that problem as a possibility. Starters (series wound DC motors in general) that have overheated will have had the insulation breaking down (that's the smell you said you have) on the armature, and like any conductor that overheats, the resistance is higher, which means it requires more current to do the work (Ohm's Law at work here) and thus the slow/weak rotation and torque. Given the history of your starter motor and the symptoms you just described, if it were mine I personally wouldn't second guess it and monkey around with the starter, just replace it.
