I'm wondering if you have made any progress. I saw on your other thread that you have previously turned the crankshaft with a wrench. You might want to measure the torque required to get the crankshaft to turn and post it here.
DC motors (such as starters) have maximum torque when they are stalled that drops as the RPM of the motor increases. I've read that the stalled torque is 3-4x higher than the torque once the engine is cranking, and an engine with plugs might need as much 150 ft-lb of torque on the crankshaft to get it turning, but only 50 ft-lb to keep it turning at 100 rpm (engine). Starters are often geared down by 10:1, so the torque from the motor only needs to be 1/10 of the crankshaft torque.
I've also read that newly rebuilt (still on the bench) turning torque of crankshaft and pistons (without valves) is ~30 ft-lb, but this drops after break in of the rings. Also, adding the camshaft(s) torque is significant (which I forgot to consider when I told you 20 ft-lb) since the camshaft lobes push against the springs, based on the profile of the lobe, tension on the spring...