Well I'm not familiar with Monarch. I did a web search and didn't find anything, so I can't say if CVA will fit.
Too many variables sight un-seen.
CVA made 2 different locks, one with a rounded back, one with a pointy back.
The tang can be different, they made a bolt through solid breech and tang, and a hooked breech.
They had available different width barrels.
CVA and Traditions used the same makers for many years, and many of those parts are interchangable but they are still type and style specific.
Don't loose hope yet, perhaps some one will come along in a few days, know the maker and give you more info. In the meantime go to ebay muzzle loader stuff and cruise the item for sale paying attention to the photo's of locks and barrels and find stuff that looks the same. You'll get an Idea of the differances available and primary costs,,you can look for free!
ebay,,sporting goods,,blackpowder,,,