Well, I finally got around to it. I found a vintage muffler and adapted it to the Viking.
1) Stock muffler
2) Milling the stack slot - .05 clearance all the way around
3) View from outboard
4) View from inboard (tape was a spacer, it burned away)
5) Attachment straps. The muffler is attached to the airframe, not the engine
The hole in the rear of the muffler is approximately .5 inch diameter.

The muffler works very well. No sign of power loss or overheating.
I would guess that the noise level is cut in half but hard to tell without a SLM.

Bob Z.

Jim - your muffler looks great. I wanted to do something like that for the OK but here's the problem.
The OK muffler is made from sheet metal. It attaches to the engine with the same 6-32 screws that hold the cylinder onto the crankcase. I did not want to add any additional mass as I was concerned that it might overstress the hold-down system.
Secondly, since the OK tends to run hot (no head fins), I wanted to keep the restriction to a minimum. The tubular muffler that I fitted has a relatively large volume as well as a large discharge hole. I wanted to allow the exhaust gases to expand as much as possible, thus dissipating energy while at the same time, minimizing back pressure.
In the future, I plan to add head fins.

Bob Z.