Might as well drain the Oil. See what kinda chunks you find.
And check the valves. You may have dropped one.

1)It may be that you set the valve clearances wrong, if you followed a Clymer book they say to rotate the motor clockwise ,which is incorrect ,you should rotate the motor the same as the wheels turn . 2) I'd also junk that "some kind of shorty muffler" for something more standard that you can tune the carb to.1) may have caused your valves not to seat or to bounce ,this could make the exhaust valves over heat as they hang out in the hot exhaust and need to seat to give up heat to the head ,thus cooling.  2) At the beginning of the intake stroke with the piston moving up toward top dead center ,the intake valve is opening but the exhaust valve has not fully closed. This is valve overlap and could have caused hot exhaust gases to explode the fresh incoming charge before the piston reached top dead center (engine knock) ,causing the damage to the sparkplug ,or valves or even putting a hole in the piston dome ,ring damage ,or whatever you may find. A unknown ,thus non -tuned, exhaust system may have contributed to this factor at certain speeds.I'm hoping you don't find any of of this, and wish you good luck!