FYI, you may want to soak the breech end in Kroil oil for about a week. During this period pick up a T/C 15/16th's breech plug removal wrench (example on link).
https://picclick.com/Breech-Plug-Rem...131501267.html
Using this tool and a good wrench you should be able to spin off the breech plug. With that you can give it a good cleaning or have the barrel rebored or relined.
New breech plugs: https://www.trackofthewolf.com/list/Item.aspx/667/1
If you are unsure on doing that job, I still would recommend Bob Hoyt on doing the work.