There is nothing super special about a Kohler muffler
A std CH20 muffler looks to be around $ 300 US new + lots of used ones on evilpay
IF your exhaust pipes are OK then a Std B & S muffler is about $ 100 and then you can learn to use that welder you bought 10 years ago that is gathering dust on the top shelf of your garage .
Looking at the mufflers shown against the part number Star posted it looks like a bog standard high level Kohler muffler as found on thousands of BobCats & other skid steers , Dingo/Kanga stand on and a host of other gear with a different mounting bracket & fancy bent outlet pipe
Not a difficult job for any muffler shop to convert
Alibaba is swarming with very similar GX 620 mufflers & heat shields as they are also used on a host of diggers & loaders but at that price probably std mild steel .