If the powder has been in sealed cans, it should still be good for many years to come.
I'm not a big fan of elephant powder because of how much fouling and overall performance is lacking but what the hell, you have it and in a percussion rifle it works ok.
I'd shoot some to see how it works. If it's alright, then shoot it up. :thumbsup:
Going forward, Swiss BP (The best BP I've ever shot and my standard) or Goex black powder.
Comparatively, Goex is as a distant second compared to Swiss but also less expensive and is a really good powder.
For substitutes, I would not waste my time or money with Pyrodex. Filthy, smells terrible and is really corrosive.
You will much prefer the following substitute powders for a percussion:
Black Mag'3, Hodgdon 777 3FG or Goes Pinnacle 3FG (I've not tried this yet, but preliminary reports are good).
:imo: