Seems obvious why 8)
For anyone looking to mod the muffler, there is Jetex Exhausts Ltd.
Standard boxe are still availeble cheap, which means that the inlet and outlet is also availeble cheap; you simply cut them off.
Now find the muffler you want (there is even a one in, two out) from Jetex and weld/have them welded together.
(https://jetex.co.uk/wp-content/uploads/2019/03/u346351r-1.jpg)
I myself have been faffing with motorcycle non-mufflers welded in between the OEM stubs. The first one melted/burned, the second one is LOÚD.
Teaches me about being cheap and knowing better: it would have been faster/easier, almost cheaper, certainly better looking and harldly heavier buying
(https://i.ebayimg.com/images/g/G5UAAOSwnoldTour/s-l500.png)