According to the instructions I received with my motor [Raw Motor] the mounting of the motor and proper alignment of the drive train are the most important part of the build. I must admit, I tried to mount my motor incorrectly and I did suffer the same fate as you. And my chain came off a lot of times. I was scared for a long time to open up the throttle all the way for fear that my motor would pop loose again, or the chain would come off again.
A mechanic removed and installed a new front motor mount stud for me and I switched to regular bike chain. All is well at 35+ mph. I drilled the frame though. I asked my mechanic to check the torque and he said it was not necessary. But he checked the head bolts anyway, all were still 12lbs. Sometimes I wonder if all of these quirky checks are actually individual engine faults or eccentricities.
I wish you the best mounting your engine and drive train. You'll get it, just be careful.