Ask around at your marina/boatyard and find a local auto electric
shop who does motor
repair. They can usually adapt similar brushes
to fit by grinding down the sides to slip in the brush holder.
I did that with my long discontiued Simpson-Lawrence windlass
. Cost about $200 including new bearings but was much cheaper than a new windlass.
BTW, that motor was British made and the company was still in business and has a website. They never responded to an email