Don't use steel wool! It can burn under certain conditions; your kart might leave a trail of destruction off-road. In a safe place, try lighting some with a lighter or apply 12v to it; great emergency firestarter.
Mufflers use fiberglass packing for effective damping. You need the right kind of fiberglass for the job, not pink insulation for example. Why reinvent the wheel, get a small cherry bomb or thrush mounted securely to the frame and run a header to it.
On one of my 6.5 Yerfs, I ran a 1" header pipe curving down about 18". I pinched the end closed, drilled a number of ~5mm holes in the last 4" or so, and welded a piece of 2-1/8 automotive exhaust pipe to that, fabricated 2 funky 90 degree cut and weld bends that resulted in 2 "chambers" along the pipe, and terminated it with a Sparky spark arrestor. The result is fairly free flowing, and quieter than stock. It was a quick fab on the last day before a trip.
If you run a small pipe into a big pipe without any further chambers or baffles, it is likely to not be very quiet.
One thing that is very important is, make sure custom exhausts are well supported. Hanging more than just the stock muffler on the two exhaust bolts alone will break them, and/or strip the holes in the engine; you don't want that!
The kart I made at age 15 was a ~9hp Briggs flatty. I welded up an exhaust that included a full sized car or truck muffler (with perf pipes, chambers, AND fiberglass) which I got from the trash at a Midas muffler shop. This made the exhaust almost whisper-quiet, and revealed the fact that the rest of the engine makes a lot of noise too! If space and weight are no object, this is the quietest way to go.
A flowmaster/magnaflow large baffle muffler won't quiet a small engine down much at all.
