For a muzzleloader, I'd look at a low-power, fixed magnification scope, like a 2.5x or a 4x.
Muzzleloaders generally aren't long range shooters, so higher magnification won't be worth all that much to you. They do kick, however, and the stronger internals of a fixed scope would serve you well.
You can spend < $20 on a fixed scope, or you can spend over $1,000. If you give us a budget, we can give you some more specific recommendations.
Good Luck!
FC