JB weld?
You may jest but look,
Yes that is a piece of corrugated metal glued to the inside of the piston!
It's from an Aprillia Ditech fuel injected RS50 scooter I bought off a youth years ago as a none runner......
Well I got it running at first but it wouldn't stay running as the head gasket was knackered as he'd reused the old one and the coolent was putting out the flame!
I striped it down and found that piston, I cried with laughter when I saw it. I think it would have worked if he'd fitted a new gasket!
You are looking at this totally wrong. That piston is fine it is the 3 others that are incorrect. Just remove them from the bore and tap into a similar shape as the other one. Might need a fairly large tapping stick.