there are exchange mufflers from a company called “SITO”.
Style is more or less stock, sound as well. You may find some at ebay.
There are other mufflers offered by a german wholesaler called Louis, http://www.louis.de
but shipping cost to you may be exorbitant.
I may help buying them locally and ship them to you, but this will add shipping cost as well. Hope you can find offers without high shipping cost.
These Louis exhausts are very quite, at stock level. For sure, they do not produce unnecessary noise, but may be disappointing if you like to hear the pure “stroke” of a Twin.
Long exhausts (no LTD style, but sounds good)
PS: Watch out for Youtube videos, title starting with “Kawasaki Z 750 Twin sound”. I’ve done short videos with different exhausts, one is with the Louis universal conic short.