I'm not entirely sure if there is a specific 'Naruto is War Machine' fanfiction. However, considering the fact that fanfiction allows for a great deal of creativity and the combination of different universes, it's very likely that such a concept has been explored by some fans. Some fanfiction authors enjoy creating unique scenarios where characters from different shows or comics interact in strange and exciting ways, and this could be one of those cases.
Yes, there might be. The Naruto fandom is vast and creative, and it's not uncommon for fans to create crossovers like this. You can try searching on popular fanfiction websites such as FanFiction.net or Archive of Our Own.
There could be. Fanfiction writers often come up with really interesting and unexpected crossovers. Given the popularity of both Naruto and the Iron Man (where War Machine is from) series, it's quite possible that someone has written a story where Naruto is somehow related to the concept of War Machine. You might need to dig around a bit in the fanfiction communities to find it.