There are hooked breech components in the Track of the Wolf catalog that you could use if you were planning on replacing a breech.
If the rifle has a traditional breech and you want to convert it to a hooked breech you should first figure out why you want to do that. I certainly wouldn't! But you can saw off the tang, file the breech plug lug into a hook and then forge a tang from mild steel flat stock by bending it over to from an L-shaped part with one leg the tang and the other leg a breech face, cutting a hole in the face to receive the lug of the breech plug and then filing the other leg of the L-shaped tang to fit the tang mortice.
I put a hooked breech on a smoothbore gun years ago when I was young and foolish and still infected with the delusion that guns with traditional breeches were hard to clean. Once I learned how to clean a long rifle I concluded that taking the barrel out of the stock was, how shall I say this politely, utterly unnecessary.
I don't recollect seeing the referenced article.
Best Regards,
John Cholin