I just typed in " Slip on dirt bike muffler", in the search engine on ebay. Threw thousands of different ones at me. I just linked the first one that looked cool.
Oh and make sure the slip on part is the right size. That's really all you have to worry about, inlet diameter clamping ability.
Like this? It wouldnt be too restrictive with the mz65? I mean the mz clone is like 70$ and the slip ons are 50$ it would have to boost my too speed 5kmh at least to get me to spend that :/