Killing an elk is a difficult business in the first place. Limiting yourself to an obsolete weapon with which you have no experience is increasing the difficulty. The standard 45-70 will undoubtedly kill an elk if the shot is well placed. Placing a shot well at 200 yards with iron sights is a skill to be developed.
Ready made ammo can be purchased all the way up to 45-120. Whether any normal human being can shoot such a load in a carry rifle is another question and is beyond my knowledge.
http://www.buffaloarms.com/.42_.45_c...n_pr-4443.aspx
http://www.tenxammo.com/tenx_ammo_B.html
The problem is that a rifle may not shoot well with a particular brand of ammo and when you only have a few choices it is really hit or miss.
Shiloh Sharps makes some beautiful rifles.
http://www.shilohrifle.com/model1874.html