Great looking plane! I have one still in the box sitting in the spare bedroom.

Is your engine soft mounted or on some standoffs. I have seen a gasser that would shake its muffler right off because its mount allowed it to shake too much.
You could also try sealing the bolt threads w/hi-temp silicone. Clean the muffler bolts and exh port threads good first.
If that don't work, red lock-tite may. The red will release at a temp. of 500F according to the inst. on the package I have here.