'94 Onan DKD Spark Arrestor Fitting Problem

I have a 7.5KW DKDFJ s/n J943244515 in a motorhome I bought about 5 years ago. I tried to service the spark arrestor and only ended up rounding the 4 sided head on the bronze fitting that I was trying to remove. Long story short - I have drilled out and ground off most of the brass fitting not knowing any better way to proceed. My plan at this point is to continue to drill and grind off as much of the remaining brass as possible and then use a small mapp torch to melt enough of the brass to expose a few of the threads on the steel female portion of the muffler spark arrestor fitting.

My questions are:

(1) Am I likely to damage anything the way I am going?

(2) Is the interior of the muffler just baffled voids?

(3) Is brass the standard material for these fittings or am I just lucky?

Thanks for your input.