I would just get a new jug and be done with it if it were me.
All of these engines have a few flaws, you just got one that had one in a bad area, that jug is really a cheap part and even after buying another one you still have a very inexpensive engine.
Here are some links for the jug.
Cylinder Body 66/80cc
Bike Motor Cylinder Body - 49cc Parts - Motorized Bike Cylinder Body - 49cc Parts - Bike Motor Cylinder Body - 49cc Parts - Motorized Bike Parts
Motorized Bicycles Gas Bike Engines Gas Powered Bikes Motorized Bicycle Parts
If you get a new jug/cylinder i'd also replace the rings that way it can all start new and seat properly.
Just what I would do and my opinion nothing more nothing less.
Good luck and sorry to hear you've had this issue with BGF, he's always worked good with me, but then again I've never had an engine from him with any problems like yours either.
As many of these things as he sells you'd think he would just happily send you a new jug and a set of rings and appoligize for your trouble and make it right.
Why is it that some of these sellers out here don't realize that all they have to do is treat people right and it's a win - win for both parties in the long run?