This a long shot but when you took the starter apart did you check the brushes contacting point on the (commutator). Yellow (A) circle in photo.
The brush contacting point (commutator) should be shiny copper looking. If not, use 00 sand paper to shine-up commutator and clean out dirt in separators. Then check brushes (B 19) for wear, if brushes are touching brush holder replace brushes or buy new starter.

The only other thing is, does the engine crankshaft turn.

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