If you can't find a proper grounding brush, use a braided copper ground strap draped over the shaft with a weight at the bottom to assure good contact with the shaft surface.
https://www.amazon.com/dp/B01M7NP6KH
or buy a roll of bulk woven copper grounding strap and solder one end for the terminal, and put a fishing weight on the other and lay it over the prop shaft.