RE: OS 55AX
If the pictures are close, then the connecting rod seized to the crankshaft and then was twisted off. The crankpin shows bluing caused by high heat. The high heat can only be caused by a lack of lubrication in that area. Something happened to the fuel supply of the engine, making it go lean at high RPM. The resulting low lubrication could not keep the connecting rod from rubbing on the crankpin. Eventually, the friction caused enough of a heat buildup that the rod seized. As the crank continued to turn, the rod would try to turn with it, but since it couldn't, the lower end got twisted off. The brass bushing would be shattered into small pieces, and scattered throughout the engine, along with the aluminum of the lower conrod end.
The only repair is to replace the piston/liner assembly, connecting rod, and rear bearing. The crank can be cleaned with 600-grit wet-or-dry sandpaper. Unfortunately, this is very close to the replacement cost of the engine.