Just a little update to this for anyone else who may be interested, so the volumetric fog component does not seem to currently be in this UEFN build so as far as disabling the time of day manager and using the environment light rig to make your own personalized sky goes you’ll probably be out of luck, you should be able to make your own custom skybox though if you know how (you would just be missing the same clouds the volumetric cloud component would provide).
That said, rather than using the environment light rig you can use the you can use the “day sequence” device to customize your sun/moon/clouds. You’ll still be using the chapter 4 skybox though and there doesn’t seem to be an option to change the skybox to any Fortnite provided skyboxes you would find in creative 1.0.