I do not have the direct answer to your question, it would be too long, however, the first two cars I owned had a starter-generator set up. The first one was a 700cc DKW model 1938 and the other one was a 1949 DKW, sort of a post war successor. Both were two cylinder two strokes. It had an outside armature on the crank shaft with a set of brushes on a disc type commutation. Once the engine was running, it worked as a generator through a separate exciter winding in the stator, controlled through the voltage regulator. The windings on the armature were used as starter and generator. There was frequently a problem with cranking, because it was loosing torque with the slightest voltage drop in the system.
I do not believe that such a system could be built competing, at least weight wise, with a gear reduction starter motor. As a mater of principle, a high torque electric motor is always heavier than a high speed motor producing the same torque through gear reduction. And, of course, at the same hp.