Have you taken apart muffler to see if it need repacking? That might be all you need. I prefer to add more fibreglass than shown here (and looser packed) so it fills entire space between centre tube and outer casing:
https://faq.ninja250.org/wiki/Repacking_a_Muzzy
One variable across all mufflers is internal volume. I suspect Muzzy single muffler doesn't have as much volume as 2 factory mufflers. Straight-through design also dampens less noise than 3-chamber muffler that makes exhaust go back and forth through baffles.
So what you want:
- muffler with larger volume than current Muzzy
- 3-chamber design rather than straight-through. Simple way to check is look through centre and you shouldn't be able to see light at other end.
- ideally easy to disassemble to replacing packing material, but you can always drill out rivets and re-rivet to assemble.
Be careful about upsizing too much as that may let too much noise through. Perhaps single muffler from EX500 would work.