don't wait until the last minute. These rifles have a habit of taking a LONG time to build. Plus, you will want to put in some though as too style and period and what sorts of features you do and do not want.
There is a rather nice Hawkenish styled .58 for sale on the net right now. Not mine or anyone I know.
https://www.muzzleloadingforum.com/...and-hunting-gun-engraved.121774/#post-1669499
It's not a completely historically correct rifle but it is an interesting one and suitable for taking down a buffalo.
Building one from scratch is the way to go though. Endless possibilities.
If you aren't familiar with them, you might check out
https://www.trackofthewolf.com/List/Category.aspx/485. They sell parts, but they also sell completed rifles that would give an idea of the possibilities.