The Great Plains Rifle is Lymans standard production muzzleloader. Chances are your rifle is less than a few decades old. This is the muzzleloader I personally use.

My personal load is 415gr TC maxi ball (self casted) 110 grs of pyrodex powder and standard no11 percussion cap from cci. Of course bore butter as bullet lube.

It will be slower than your 50cal of course that means losing tragectory faster. But well capable of taking on longer ranges with the experienced hunter behind it. It will hit harder than the 50cal in terms of energy displacement. But either the 50 or 54 is plenty of bullet for most game in america.

Also if the gun is in good shape and no rust pits in the barrel let me know if you decide you don't want it.