Have your dad's friend balance the crank, have him inspect/replace the bearings with better quality, and have him inspect the casting to make sure there aren't any flaws that need to be cleaned up while the motor is apart. (Make sure he knows the QC on these motors is dicey at best and he should inspect for poorly finished parts!) Have him replace all the studs in the motor with higher quality studs when he rebuilds, that should give you a nice improvement in smoothness and reliability compared to running the motor right out of the box. None of these parts will cost you very much, if you have a qualified mechanic willing to do the work for you I say go for it....
As you have learned already, you can't really bore the cylinders out on the china girls without getting into expensive plating. Best to either run them stock or perhaps tryout a moped cylinder....