What part number ?
the simplicity one or the B & S one
An engine that old should that a NPT threaded exhaust port
From memory a 190000 series is aroud 10 Hp and has a 0.75" or 1.0" NP thread
Basically it is a galvanised iron water pipe thread as previously mentioned
So you have a choice of a threaded muffler or a bolt on muffler
On the web page below are the B & S mufflers that should be able to eiter screw in or bolt on
The last item is a deflector that is screwed , or riveted over the outlet holes and pointed down to deflect the exhaust which makes the exhaust a lot quieter .
Now these are what was fitted to the cheapie mowers that used both the std B & S muffler and usually a B & S fuel tank strapped to the side of the engine .
If you click on the MORE box then you will see the B & S part numbers .
An muffler with "low-Tone' or "Hush Tone" wer the quieter ones
The page linked to below is of course from one of my Wholesalers in OZ but if you go to the Rotary, Stens Prime Line web pages and search Mufflers then you should bring up a similar page
After Market B & S Mufflers
Also on most of the sites is a "dealer Locator "
I know you will find this hard to believe but there are more places to shop than Scamazon & Eviplay
The muffler does not have to be exactly the same shape as the original but of course must be able to fit your mower
As you did no pos either your mower number or better stil a photo this is as good as I can go.
Generally a JD muffler will be quieter then the Murray mufflers followed by the generic B & S ones .