From scratch? No, consequently there is a pretty much automated aircraft converter [1] and the directory structure is incredibly similar and early alphas had the fsx flight physics, and started with fsx airports before the ml scenery was finished.

It's certainly a more ambitious engine upgrade of the engine than p3d or fsw, even excluding the cloud scenery stuff, but "write a flight sim from scratch" is a task on par with "write a Web browser from scratch".

Yes, most of this forum could write a IE3 or Netscape 2 or Lynx clone in a long but manageable amount of time, but if you want to catch up to Chrome/Firefox/Safari, good luck.

[1]: https://www.nexusmods.com/microsoftflightsimulator/mods/117