Sounds like he wants to core a stock exhaust and install a inner perforated core and exhaust packing vs a stock turbo style. I removed them with a metal chop saw and had retaining spigots welded internally for the new core. you will need to drill holes for new rivets to hold it all together but you can pick your size. I highly recommend blind sealing rivets so you do not get exhaust coming out through the rivets
on the 851 muffler -a04, b04 - you can grind out a spot weld on the inside of the large outlet and the baffle slides out. makes them very loud and harsh, but that was the "racing mod" for the std mufflers as given in the "modifications for racing" put out by ducati in 1989.
the 888 muffler - maybe a/b11? - doesn't have that.