Here is a technique that I have used successfully for holding brushes while assembling certain motto types. Make a two inch long piece if cardstock about the width of the spacing of the brushes. Make a hole in the center for the shaft of the armature. US the cardstock to compress the brushes into the holders and wrap it around the plate, holding in place with Scotch tape.
Place that assembly over the armature shaft and start the brush plate screws just enough to hold. Then tear out the cardstock, which will leave the brushes in contact with the armature. Then tighten the screws.
