Have you read the unreal engine license agreement? [0]. Section 3.a covers modifications
[0] https://www.unrealengine.com/en-US/eula/unreal
Edit: I see some language covering this further up.
Have you read the unreal engine license agreement? [0]. Section 3.a covers modifications
[0] https://www.unrealengine.com/en-US/eula/unreal