Don't worry about the cracked insulation- - - -it's not all that uncommon. Be sure to carefully check the clearance of the bushings on both end frames- - - -worn ones will allow the armature to drag on the field coil pole shoes and slow the speed down, and also create metallic dust inside the housing. Inspect the commutator bars for any evidence of burning on the edges- - - -that indicates a shorted winding that's making contact with the armature laminations. The bars I can see don't show any evidence of that problem.

New brushes and bushings, plus a new pedal switch should put it back in good shape for a long time to come. The best thing you can do to assure long starter life is keep the engine well-tuned so the starter doesn't have to run for very long each time it's used.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!