I do not know about the spark arrester, The Muffler is supposed to provide that function.
But on My ONAN when the tail pipe.... Oh pipe descriptions
Generator--Exhaust pipe--Muffler--Tail pipe--Genturi
When the tail pipe rusted out due to poor placement I went to local muffler pipe bender he bent me a replacement.. I came home and was working on the Round Tuit (No job gets done till you get a Round Tuit) but came home Thursday and spotted a workman talkign to a neighbor.. Since his truck contained welding gear.. Instead of clamping the new pipe to the old .. It's welded on. SOLID.