Piece of a gear by the looks of things, lying under crank shaft, every time big end passed the bottom of its stroke it hit this object.
Not to much damage, counter weights on crank got a couple of dings in and inside crank casing has some marks and gouges in. Obviously this piece of gear went round a couple of times.
I haven't split crankcase as yet, just got head, barrel and piston off. Once It was all off I could see into bottom of crank case and noticed it.
Use a magnet to retrieve it.

I can't seem to post photos otherwise I would.

Still have to split crankcase to check where this piece can from.

Pepsi and Pleiades you both seem to be spot on.
Piece looks just like piece of gear dog as in link.