Fateforge - 5e Game System

[Ambush]

Let's say that I'm playing in a new game for Fateforge (just released by Studio Agate). It's based on the 5e rule system, but pretty much everything is up for grabs. It uses the same ability scores and skills, but species (races), feats, spells, etc is different. Also there's the occasionally new thing that needs to be tracked like corruption level or madness.

What would be the best way to go about it? Create a new game system from scratch? Create a new source and uncheck everything but that source? I'd really dislike having to do every single thing from scratch, but if that's the best way, it's the best way...