Piston and cylinder show a classic "4 corner" seize pattern, indicating that the piston got too hot and swelled and grabbed inside the cylinder.
The brass bushing at the small end of the crank is damaged as well. What 2 stroke oil/premix ratio were you using?
If this were my engine I would replace the crank, seals, and bearings with one of these.
You can try to hone the cylinder reuse, and maybe sand the piston and replace the rings, but with stock cylinder it's probably cheaper and easier to find a good used replacement for both on the buy/sell forum or buy a newer cylinder kit.