The Lyman is a 1-48" twist. In a .54 caliber, that's a "compromise" twist; it's not a slow twist for specifically roundballs (like a 1-66" or 1-72" would be), and it's not a fast twist specifically for heavy conicals (like a 1-32" would be). In theory, and in practice, it should shoot either the roundballs or the conicals decently well, and certainly well enough for most hunting purposes. If you can hit prairie dogs with it (though, with a .54, it's about like grenades on them: close counts), you'll be fine on big game.
I haven't shot an elk with a muzzleloader, but I do know that the T/C Maxi-Hunters and the Lyman Great Plains bullets for the .50 and the .54 work incredibly well on deer, including full-length penetration. They should be fine in the .54 for elk, as would the patched roundball, frankly.
What you are going to need to do with conicals in that twist, though, is kick them out hard. Think 110+ grains of powder hard. The roundballs could well be the opposite (as in 70 grains of powder), but I've seen them work just as well with a heavy charge in '48" twists, too.
For powder, I'd run FFg or FFFg real black powder. The substitutes don't ignite nearly as well or as consistently in a standard percussion gun, and they are just as corrosize. Pure black gives you great consistency, almost no spread in velocity, very consistent and positive ignition, and as long as you clean it, you'll have no problems.
If for some reason you have a hard time finding .54 conicals, let me know. I probably have some stashed away somewhere.