You are right that they PROBABLY can't tell....but if they do....

and the logic is that the old vehicle is just the "Raw material" for the new one. Similarly, if I import a mitsu turbo, it is made in Japan, subject to duty as well as taxes. If Slowboy ports it or does the flapper mod, the turbo is just the "raw material" for the new product, and it is now an American made product, subject to taxes only.

That is also why so many large companies get all their parts made offshore cheaply, and then just assemble here, so that they can say Canadian made or US made...