Can I test a DC motor by replacing the brushes with wires?
Just touching wires to the commutator is probably not going to work very well.  The brush rigging is arranged so that the correct poles of the motor are energized together; just by randomly touching wires you are probably not completing the circuit through one winding, and even if you are, it might not be aligned with the magnetic structure.

Are the brushes badly worn?  If you can access them, pull them out and inspect them, and the springs for them.  Clean out the brush channels.  You could even go so far as to use a very fine sandpaper to polish the commutator LIGHTLY, and even wrap some around the commutator sand side out, seat the brushes, rotate the shaft, and clean and re-shape the brush ends that way.

Edit: Obviously, make sure you can turn the shaft by hand, to be sure that no bearings are seized up.