Okay, seized up.
Lift the jug at high as you can and hold it up with blocks of wood between the edge of the cylinder and the case so you can tap the piston down with a hammer and a piece of wood, as it moves, add more wood as needed. You don't want to pound the piston into the case, flywheels etc. You could also clamp the cylinder in a vise and support the case as it drops down with the pounding. The idea is to only pound on the piston and not damage the rod or rod bearings, crank, etc. Your cyinder and piston is going to be junk, don't worry about damaging them now. Once the piston pin is visable, remove the clip on one side (or both if you wish) and drive out the wrist pin. You should then have a "good" bottom end to work with. Flush the bottom end out with something to get any metal junk out of there before you replace the piston/cylinder assembly. You can use gas mix or brake clean to flush it. Be sure to oil the bearings good after that and before re-assembly. Any loose metal down there will damage the new parts, get it all out if even if you see none. Check your rod good too, it will wiggle side to side some but should NOT have any up and down play when you hold the crank and pull up and down on the rod If there is up and down play, the bottom end has damage too. Likely, you will just need a new top end and some gaskets. Check for air leaks at your carb, manifold and possibly the case gasket and crank seals, you could have had a leak and it was too lean causing the siezure. Is the clip set too high on the needle? Has the main jet been resized too small? are you at or near sea level? Maybe you just had a junk top end? Just pointing out that if your too lean, this could happen again after you replace the bad parts.