You cannot fork it and change its licence unless orig unreal license permits you to do it. Does it?
edit: it doesn't unless Epic Games grants you a specific license to do that.
https://www.unrealengine.com/en-US/eula/unreal
source:
> 4. How You Can Share the Licensed Technology When It Isn’t Part of a Product
> You may only Distribute Licensed Technology (including as modified by you) outside of a Product as expressly permitted by this Section 4.
> a. Sharing of Engine Code
> i. Sharing Engine Code with Another Licensee
You may Distribute Engine Code (including as modified by you) in Source Code or object code to a third party who is separately licensed by us to use the same version of the Engine Code that you are Distributing.
>Any public Distribution of Engine Tools (e.g., intended generally for third parties who are separately licensed by us to use the Engine Code) must take place through a marketplace operated by Epic such as the Unreal Engine Marketplace (e.g., for Distributing a Product’s modding tool or editor to end users) or through a fork of Epic’s GitHub UnrealEngine Network (e.g., for Distributing Source Code).