You may have to file or sand the brushes so they will fit in the holders

I have worked on Sullivan and Hobbico/Tower starters.

I assemble the bushes on the back plate.

Install the armature only onto the backplate and bearing. You can hold the brushes back with you fingers.

Hold the armature down and slide the field down over the armature.

Then install the front bearing plate

No need for strings or wires to hold the brushes back.