Did you remove the shims on each end of the armature?
The running problem sounds like the armature gear isn't meshing properly with the worm gear. You may have to place a shim under the motor to push the front of the motor farther down, for better contact with the worm gear.
Are re you sure the shaft bearings were damaged? Only curious as to how.
I used no shims except for the 671M-19 rear thrust washers next to the bearings as specified in service manual. (This is earlier design of front bearing that uses the smaller diameter washer)
To clarify, the motor I just described is the original to the engine, its commutator segments were severely damaged at some point and my grandfather replaced it with a later (large front thrust washer; small rear thrust washer) motor. He never threw out the original motor, though all washers and ball bearings have been lost. This most recent attempt at repair involves rebuilding the original motor hence the new armature and other components.
As far the newer motor goes:
The gearing issue was almost solved by shimming the motor a few months ago. After shimming seemed to create only the slightest difference the next culprit seemed to be a worn worm, in an attempt to remove the armature the end of the worm caught the front bearing sleeve and yanked it in to the motor housing (This is the bearing damage I mentioned earlier). An attempt to drive the armature out the rest of the way only munged up the worm.
As it happens most of the ball bearings were missing out of this motor as well.
