I did this setup on my J3 Cub running a Zenoah G38 with a pits muffler. This was done in 2000 and just had to redo it in 2023 due to the gasket was gone and the muffler had some play. I will note that the bolts could not be moved until the retainer was removed proving that it did work.
I have seen so many so called fixes on the web and tried many of them with no luck. On many mufflers the bolts are counter sunk into the muffler making it imposable to safter wire them. So I safter wired them from the outside. Take the muffler and drill a small hole (I used a 1/16 bit) into the side of the muffler stand off straight into the center of the bolt hole. Keep in mind you will need to be able to access this are to drill into the muffler bold after it is assembled.
After you have the hole drilled insert the bolts. Make note of how deep each hole is until it hits the bolt. we will call this the flange thickness.
Now make the retaining spring. Made from some scrap piano wire just smaller then 1/16 dia. Bend to the shape above so when inserted into the holes on the muffler you want the end to come to the center of the bolt holes.
Next install the muffler and I use a small bead of the Permatex Ultra Red Gasket Maker and assemble the muffler but only tighten to about a 1/16 gap. Let setup and then tighten the bolts down.
Now drill into the two holes and into the bolts. With the flange thickness from above I like to add 3/32 to it. So you will be drilling 3/32 into the muffler bolt. Now insert your retaining spring you made above. Now try to unscrew the bolts you should not be able to. If you can then you did something wrong.
You will see the small bow in the spring thats where it has compression on the ends.
