The four armature brushes are part number A-10069-AR and the field brush is the part number A-10070-AR. There is a fellow that has a set for the three brush powerhouse generator.
https://www.ebay.com/itm/14531531846...xoC9sEQAvD_BwE
You can also check here.
https://www.fordbarn.com/forum/showt...werhouse+brush
The brushes are getting hard to find. Some folks whittle down larger brushes to fit.