If the U.S. did withdraw from NAFTA, then the Canada-U.S. free trade agreement—which, as you mentioned, is currently under suspension—would come into effect. It may take a presidential proclamation to initiate that process on the U.S. side. We would probably both have to make regulatory changes to comply with that agreement, as things have changed over the years, but it is true that if the U.S. withdrew, the currently suspended bilateral agreement between us and the U.S. would come back into force.
On December 4th, 2017. See this statement in context.