>>528915775
>It was offscreen, so as far as we know, he got another mechanic to do it and didn't make it himself.
No he made it himself, that was part of his character arc of proving himself to Sonic and helping him reach the Death Egg and saving the world. Just because it wasn't explicitly shown Tails adding that engine, it doesn't mean that wasn't the intention

If you know how to fly planes, it's expected you know how planes work and function