if all else fails
take a new brush, from something bigger
carefully with a specially prepared hacksaw blade or a dremel tool
cut a groove on each side of the brush
then from a hobby shop or maybe a scrap piece of copper sheet, cut out
the backing plates and leave enough to foldup and into the grooves in the brush
then carefully tweak down the excess to trap the brush and lock it to the backing copper backing plate
tin and sweat to your old support arms.
if it were me i would make them slightly larger to get more friction area.
if i knew how to post a picture or rather a jpeg i could illustrate the process
much easier, a pic worth a 1000 words as they say.
bob g