Problems with Engines

Have you checked the condition of the armature face and the brushes on 898?  That's where I'd start.  They made need cleaning.

Where is the armature face?

Maybe I'm using the incorrect term here, but if you look at the side of your motor that does not have the gears, you'll see copper-colored segments.  Try rubbing these, rotating the armature, with cotton tips moistened with alcohol.  You'll notice that there are three segments, with smalls gap between.  Clean out the gaps with a toothpick and alcohol.

A technician overhauling the motor would disassemble it, but this should get it clean enough to determine if a dirty armature is your problem.

Probably though, the two brushes need attention as well.  They are on the same side of the motor and are app. 3/16" in diameter, located so as to rub on the armature face.  A spring holds each in place.  The spring can be pried away at the non-wound end with tweezers, a tiny screwdriver, etc.  Hopefully, you can then flip the motor so they fall out.  soak them for a bit in alcohol, then rub the non-scribed end squarely over a paper towel to dry and renew the surface.  Reinstall so that the spring seats in the scribed groove.

Like I say, this is not thorough, but should get your motor running if indeed this is the problem.