Are you sure it is a Marsh and not a March Pump? (March Series 809 is a typical pump used in hot water systems) You could try contacting the company and see if they would send you a brush cap. Sometimes they will send it free as a customer service. At the very least they should be able to provide contact information on a source of parts.
American-Marsh Pumps
185 Progress Road
Collierville TN 38017
800) 888-7167
MARCH MANUFACTURING, INC.
1819 Pickwick Avenue
Glenview, Illinois 60026-1306
P 847.729.5300
Good luck
Steve