It had a patten breech and he just sawed the nipple off and put in a liner.
If it had a patent breech, the nipple screwed into the snail and could be removed with a nipple wrench. But the nipple hole would be 45 degrees to the barrel. Most likely it had a recessed breech with a drum screwed into the side. Then you would unscrew the drum, not hacksaw it off.
Then you would find that the hole has 10mm threads, so you would have to make your own liner and unless you have a lathe you couldn't turn a shoulder on the liner to keep it in place. Also, the liner length would have to be precise, otherwise it will protrude into the flash channel and cause a fouling trap. To do that properly, the breechplug needs to be removed and you won't accomplish that without twisting the tang off the breech.....cause they don't come out.
This conversion is a fool's folly and best left alone. If you want a flint pistol, buy one. It'll be cheaper in the long run and you won't end up with a pile of screwed up parts which you'll have to sell on ebay anyway.