I thought somewhere I read a piston number the OP put up that led me to believe it was a 64mm.
You say it ran soooo great but its backing plugs out and melted down! Whats your overall set up? What pipes? WHAT CARBS AND JETS? Timing? Fuel? Compression before melt down?
Make absolute sure you or the builder do a leakdown test on this motor after rebuild. Man you have so many variables, this may not be a.good motor to learn how to rebuild on. At the minimum, send it out and get it checked over and have that builder send it back with the new parts you need to put it back together. It wont cost much to send a crank and top end to somebody!