I used red locktight on the engine bolts that came loose and let it cure for a full day. I then ran the engine again and did some additional tuning. The engine is running much better now but still has some vibration but much less than before. The bolts seem to be holding, at least for now. I may actually be ready for a maiden flight with the GP Giant Super Sportster powered by the NGH 38GF engine.
Best wishes and good safe flying.
Heli-NuBee (AKA Roger the radial rabbit)