A small air leak will not do what it's doing but more likely to lean and In most cases act up (lean) when hot. Not saying there isn't a crack in the plate OR ELSEWHERE. I would do what I said and get a spray bottle with gas and start it and regulate it this way. Abviosly pushing the choke in as soon as you spray it.
Now you have ruled out most everything.
This brings it back to the carb, filter or gas line.
You could get more precise with a drip method with an even administration.
A healthy engine otherwise you should have been able to idle it. Obviously your situation it would not have and then had you looking elsewhere. At that point pressure testing is the way to go and easy.
My Mechanic friend uses a couple of pieces of rigid rubber about 1/4 ' thick for blocks.
Then a piece of Aluminum backing that was cut of a 'Yard stick' in his case, then drilled. I understand Stihl offers a kit and its a dummy sparkplug for the air source, which makes it universal. I guy could make a fitting there. My saws are 372's so I just use the impulse line then dunk it in a tank. you can use a compressor turned right down. It's the only way to fly man.
It's good you got it anyway.
Personally I think the piston is a none issue down there. If someone says different, I may have questions but its not in my wheel house. I was thinking the possibility of the bolts backing off there. I put a 52mm BB on mine and the cylinder was always lifting. I can't say I've seen one break like that.