I would go in search of a small,thin plastic tube same diameter and thickness as old cardboard tube. Or you may have to find a piece of material you could form into a circle then hot glue to make one. I am assuming the brush holder is mounted inside the nosecone where you couldn't use anything to pry the brushes back while inserting the armature.
On question about ball, if the end of the shaft has a recess machined into it I would say you need the ball, once inserted and tightened up not enough play to let it slip out and it would run against thrust plate. If shaft is smooth on end no ball needed it would just run against the plate and only when under extreme thrust.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979