Cast iron=superior in MY OPINION. get yourself a temp gauge as well.
How did it run if the Piston diddnt slide through? And yeah, I know you rejetted when you went to 70cc. What I'm saying is that you needed to do it again after plugging the decomp.
Post some pictures of the Piston and cyl wall. But yeah it's probably toast now...burnt kits make great ashtrays tho!