Next plan as it has been suggested to me that the crank bearings might be worn allowing the piston to contact the valves leading to this mess
I dont feel this argument holds water..
Was it raised by a member or the person who failed to put it together properly... whomever that may be.
Can’t highlight as it’s on my phone and I’m old(not that bright)
Look for the strange clean spots where they look oddly clean or patchy dirty.. look for the incongruous marks.
Look at the piston to bore gap difference top to bottom
Pop off the barrel and check out the gudeon pin and skirt areas.
You’re up for a rebuild / exchange motor from the looks anyway
Pm Eltwin here .. he’s had some experience.