[VintageMA]

If the pinion is plastic and the screw is metal, you might have a hard time there. Might be time to look for a metal pinion.

Does the motor shaft have a flat spot on it? If not you could create a flat spot on the shaft with a dremel where the screw touches it - that would help.

Then you could use locktite on the end of the screw that meets the flat metal shaft of the motor. Tighten the screw down good and let the locktite set on the metal to metal contact point for a day and give it a try.