Got one a few years ago. You have two options: declare it at the border and pay custom taxes or dont declare it, maybe they won't search the car! Anyway, you'd better have a copy of the regulation about flintlocks because most custom officers don't know they don't need to be registered...
I cross the border many times a year with one of my flintlock, i can tell you the problem is always getting back IN Canada with it, never goint out!