Your pictures show quite a large gap in the pinion to ring gear mesh...so that will never be a reliable way to do it. Is this engine a P series or a B series? The starter looks to be a solenoid-shift type with the plunger fully actuated -- maybe that was needed to get these pictures. The two starter types have a different number of teeth on the pinion, so they need different spacing from the center-line of the crankshaft / center of the flywheel. Here is a statement from Boomer in a much earlier thread.
can you post some pics of this?
if this indeed a P series the starter will mount on the bottom of the # 1 cyl.
it has the 9 tooth.
the B43/ B 48 M has the 10 tooth, but it mounts to the oil pan.
the ring gears are the same.
thank you. boomer (the used onan engine parts guy)