Plus it already has been done before with Windows XP, without even any documentation and there is a guide on the internet on how to build Windows Server 2003.
This was done (if I remember right) when governments and big customers had access to the Windows source-code.
Just getting the code to compile, link, baking in assets, etc. For a single architecture is a much more reasonable goal.
I'd imagine it'd be on the scale of 1-3months for an engineer to get working full time, but large error bars around this figure