The engine in the frame in your first photo is a "TC" (dual range transmission).
You're a good man for taking this POS of the PO's hands.
From what you've posted, I would suspect the missing piston skirt was removed by the PO and the top end put back together about the time he put it up for sale.
With what you have found so far, I would be thinking about splitting this cases "IF" you plan on keeping the bike. It appears to be in good shape cosmetically...?? with the exception of the seat cover.