Re: "no requirement of 'registration' for deactivated firearms"
By that logic, the same ought to apply to replicas too. Does it?
Yes. You could make a fake firearm from icing sugar if you were so minded and if it is realistic enough for anyone to think that it is genuine when you brandish it in public then you can expect a vigorous response from the police. If you are lucky (or careful) enough to survive the experience then the law will not suddenly say "oops it's only icing sugar; sorry for bothering you".
One way or another an imitation firearm can cause just as much trouble as a real one.