Piston ring snapped and..

Disappeared into the open crankcase.

It was during the refitting of the block onto the case after changing gaskets and rings. The piston started in crooked and snapped, but was caught in place. When I lifted the block back up, I didn't catch it as it fell. I've fished out the big piece, and I assembled it together on the table and found a small piece was still in there.

So now I've removed pistons again, and cranked the ignition plate with a wrench to see better, and all of a sudden it stops turning. I wrench it backwards and the same happens. So I get about a half rotation.
What I think it is is it's stuck inbetween the teeth of some sprocket, and its blocking the chain. Does this sound about right to you guys? Or, given this evidence, does anyone have a better idea where I should start poking my magnetized wire which I'm bendong into shape as we speak?
I'm going to try search gingerly and carefully before I commit to extracting the engine and flipping it.
Send me all your luck

 

Which piston?

 

#3, and I fished a large piece out of the cam chain tunnel. I know there's a big gear in there, and deeper down there is the drive chain. My suspicion is its there that the piece is stuck. I will check when I get home.
I think I should get the award for largest nimrod of the day, as I was supremely careful about cleanliness up until this point.

 

Odds are that the bit of ring did fall onto the gap between the primary chain and the primary chain sprocket on the crank. If it had fallen behind a crank web it should have slid down into the sump once the crank was moved.