When the starter was converted, did they do a full rebuild? ie; bushings, brushes, turned and undercut commutator, Bendix replacement? If so, I'm inclined to point you toward a faulty ground point.
The negative battery cable should be attached to the front of the battery tray. This connection point should be clean bare metal. Additionally, you should have at least one ground strap from the frame to the engine. This is usually from the timing cover to the frame at an engine mount. All connection points should be spotless. Then, the mounting surface of the starter to the engine has to have a clean metal to clean metal point also.
I don't know if this is your problem, but this is an easy step, and is worth the effort.
The clutch dust theory just seems improbable. It would take a lot of debree to stop the Bendix gear from engaging the flywheel. Is the cleaning done to the gear area of the Bendix or the motor windings inside the starter housing?
Tim