OS recently updated their 4-strokes, but before the new ones came out, we rarely ran a muffler at all - just a straight pipe with no pressure.

The newer ones require pressure - they don't run worth a damn without it.

Now, I'm not familiar with the Magnum 120, but I would say that if it were me, I would at least TRY running the engine with no pressure (You'll need to richen the mixture a bit, but that's all).

If it works well with no pressure, then you can do whatever you like with the muffler baffle