I'm putting my 64 Chevy power top motor together (had fluid leaking through the motor) and it's been 50 years since I assembled a small 12 volt motor. I forgot the trick or tool for holding the spring loaded brushes so I can slide the armature back in. How do I hold the brushes in?
Can you put the armature into the brush end of the motor, before assembling the field housing to it? If so you might able to make a tool from wire, that will hold the brushes both back in the holders while you insert the armature. Or see about making a washer that will fit into the opening between the brushes, and then stay in the area between the commutator and the bushing, after it's assembled.
Look on the back side of the end piece for small holes to install stiff wire to hold back the brushes . Or maybe on the sides of the holders to the side end piece . I used paper clips but that was along time ago.
I've taken apart some old Delco motors, they don't have holes for wires to hold the brushes in place. But that doesn't mean you can't make your own holes.