Based on your experience with intermittent starter motor starting, I suspect one or both of the overrunning clutches are sticking. The main one is between the water pump and the generator end input shaft from water pump. There is an oil cup for this, but passages may be full of crud. Getting at this clutch requires removing the water pump, which is a chore in itself. You could try adding some light machine oil to the oil cup at the front end of the starter/generator.
There is another overrunning clutch inside the cluster gear that is shifted by the starter pedal.. There is a grease fitting on the end of the cluster gear shaft, accessible by removing front floor boards. When floorboards are out, it is a good idea to put one, maybe two shot of grease in the clutch release bearing, accessible under the clutch access cover. Do NOT overgrease this bearing, excess is just thrown out by centrifugal force into the bell housing.
Another possibility solved by the hammer blow, would the starter brush lifting mechanism sticking. That linkage is all inside the rear end of the starter/generator. There is some access to these brushes through frame openings under the cover band.
The hammer fix makes me think of the sticky brush gear. The 4 starter end brushes are normally lifted off the mating commutator by a cam mechanism operated by the starter pedal. When the pedal is pushed down, the brushes are lowered into operating position on thee commutator. When the pedal is released, they are lifted by springs.