If it is a sidelock, traditional style, you might be able to drill out and tap for a musket nipple....hopefully the hammer will still be large enough to smack it. If it is an inline, you'll need a new breech plug.
Older models of traditional rifles used a drum screwed into the barrel and the nipple was screwed into it. Replace drum and get a new nipple.
Need more information to accurately prescribe remedy.
Been awhile since I was there but Dixie Gun Works used to sell oversize nipples for just this reason. They are usually standard tap sizes so you just retap with the next larger size and put in the new nipple.
If the threads are truly stripped out, going up a size or two on nipples won't do the trick as the nipples are sized up in about .005" sizes based on pitch diameter. I've used them to replace nipples that got loose but not for stripped out threads. 1/4-28 is the standard nipple size for sidehammer muzzleloaders. Might be someone making 5/16" (musket size thread, I think). I've had nipples for musket thread that take a standard #11 cap. Depends on what you have left for material to work with. Don't compromise, regardless of what conventional wisdom says, these guns work at the same or higher pressures as modern shotguns (over 10K psi). Nothing to screw with, no pun intended.
Did you strip the threads out of the barrel or did you strip the threads off the nipple? I have stripped the threads off things before and the bolt or male item lost the threads so all I had to do was to clean out the hole and those threads were fine. Get yourself a tap and see if you can't clean out the barrel thread of metal. it might work and be cheaper.
If it is a 1/4-28 nipple, Dixie does carry 5/16-24 nipples which are used on the Enfield rifle. They also have nipples in .260-28 .265-28 .270-28 .275-28 and .285-28 The trouble might be getting a tap. You might get away with .005 OS if the pitch is the same. Without anymore info on your problem we are just shooting in the dark.