years ago in "81, A friend had a yamaha piston fitted into a PUCH High Torque cylinder ( tranfer walls were Real thin and cylinder ran REALLY HOT! which pistion? I can't remember for sure. but, I think it was a 1 over piston from a yamaha 2cylinder 100cc.
2 years ago, I did get my Zeta 65 bored over with a rare 1 over zeta piston ( went to Cahill's)