I just don't think this is starter related. With the spark plugs out he should easily turn the engine by hand using the fan. Even with the plugs in it should not take a lot of effort to turn that engine by hand.
My 261 turns by hand easily enough with the fan and it runs fine.
With a 6 volt starter and flywheel that matches if he were to apply 12 volts to the starter it should really make that engine whirl over. You can do this providing you don't just sit on the starter. You can burn it out fast if you crank for any length of time using 12 volts on a 6 volt starter. But I have run a 6 volt starter on 12 volts with no problems.
I'm still betting on a tight main bearing.
