your pictures show a typical lean mixture seizure. If it were oil related the seizure would be at the bottom of the skirts. oil lubricates the bearings, fuel cools the piston crown.
The question I ask when seeing this type of piston, is where were you holding the throttle the first time it quit. The leanest jetting is a 1/3 maximum speed. Your butterfly carb regulates this area by the holes drilled in the body (under the welch plug) and not by any of the fixed jets (unless you count the fuel that must pass throught the pilot jet).
Also, poor fuel quality (old gas) will cause this same "4 corner" seizure as it does not vaporize well, thus the heat is not disipated. Top of the piston, heat. Bottom of the piston, lack of oil or snow injestion.
I see from your pictures someone used RTV. RTV should NEVER be used. That was OK in the 70s, but even tephlon (sp) tape can plug an oil passage on a modern 4 stroke.
"The Works" toilet bowl cleaner and a Qtip will work well to remove the aluminum from your cylinder. Do this before you take it to your local shop to have the bore checked and #1 they will have a better estimate of the damage and #2 they will not charge you for acid washing out the aluminum. If the aluminum is there, it will tear up there honing stones, so they will charge you to remove is with acid first.
