Elmer Keith said scopes don't belong on handguns, no doubt referring to revolvers, but it was already being done fifty years ago--even a hundred years ago, though not commonly. But if you must do it, the barrel has to project out beyond the end of the scope and you may have to be creative with holstering (or sling).
I've never seen a photo of an old lever action with a "period-correct" scope but that doesn't mean it wasn't done. The problem at the time would have been the top ejection and side mounting of scopes is not common. However, telescopic sights were in use during the Civil War. So if you wanted one, one of those long scopes, if you can find one, would not be wrong. It would still be unusual and it might look a little odd but it wouldn't be wrong. I suppose you could go to the same trouble if you wanted a scope on your '03 Springfield, too.
I've never seen a photo of an old lever action with a "period-correct" scope but that doesn't mean it wasn't done. The problem at the time would have been the top ejection and side mounting of scopes is not common. However, telescopic sights were in use during the Civil War. So if you wanted one, one of those long scopes, if you can find one, would not be wrong. It would still be unusual and it might look a little odd but it wouldn't be wrong. I suppose you could go to the same trouble if you wanted a scope on your '03 Springfield, too.