I remember the Browning Mountain Rifles, they were a very well made firearm of very fine quality. You are very lucky to own such a fine quality replica of blackpowder history. But inorder to accurately answer your questions and offer consistant advice there is some information that I need about your firearm.
What are the dimensions of your barrel? For example: My
CVA Mountain Rifle is 15/16 across the flats and has a barrel length of 32 inches. The barrel is a hooked breach and if I were to replace the barrel I would need a replacement barrel of the same dimensions in the new calibre desired and possessing a hooked breach. So when I buy my replacement barrel, I would then need to install the hooked breach plug into the new barrel, or ask and make sure that the breach plug is installed at the company that I bought the replacement barrel. Then upon recieving the new barrel I'll need to drill and tap the barrel for a Percussion Drum or a Touch Hole, depending on whether my firearm were percussion or flint. Next is to install sights, followed with tennons to hold the barrel to the stock, and finally I would brown the barrel....because I like the look and feel of a browned barrel. Once all this is done, I could swap barrels/calibres whenever I wanted.
If I am very fortunate I could find an aftermarket manufacturer that sells drop in replacements....But I know of none for the Browning Mountain Rifle.
Your choice of a .54 calibre for elk hunting is a good solid choice. I've taken many elk with a .50 calibre, but when I started hunting with a .54 calibre, I found very quickly that the .54 calibre possessed much more terminal energy and shooting a conical bullet just made things even better. Being limited to open sites here in Colorado, I limited my shots to less than 100 yards. It defeinately put the hunt back into hunting...