Check your provincial regs on smallest black powder caliber that can be used for big game. If they allow .45 cal, then use Thompson Centre "Maxi-Balls" or try the Lee R.E.A.L. conical bullet.
What you're looking for is a conical projectile of around 300 grains (the Maxi Ball is 320 grains). I would not recommend a round ball, but a 300+ grain conical should be adequate (it certainly would be considered adequate if you were using it in a 45-70, so why not a front-stuffer?)
My personal preference is .54 cal, but heavy conical bullets in .45 cal should work.