The way these Torrington type clutch bearings work is this. The rollers are forced to go up a short shallow ramp by a small plastic spring behind each roller. The roller locks between the ramp and the hard inner race when the prop is turned back wards. The usual cause of slippage is when the plastic springs get weak or disintegrate due to heat and/or age. Not using the proper grease or not replacing the grease often enough can hasten the wear.

I have never seen a G-38 in reasonably good condition that would not easily hand start. I had 2 on my B-25J.

Bob, I'm sure you can get the crankshaft nut from B&B Specialties.
http://bennettbuilt.com/page4.htm