This video show how you can make clouds for your game in just 2 min.
Skill level:  understanding the basics of Houdini and Unreal

With Houdini you can convert base shapes or any other geometry to volumes. On this volume noises and other effect are applied on it to make it more looking like a cloud. With 3-4 nodes in Houdini can already have some nice clouds in your viewport.
After the cloud is made, you will see how to create a simple mask for that cloud. With placing lights in Houdini you can create this effect.
There are more ways of doing this and even getting direction mask for more complex materials.

In Unreal you will see how to make a base material and use the mask to control different colors of the cloud. This can help you fine tune the cloud for the scene.

More detail view on clouds in Houdini https://vimeo.com/200651074

An overview of the Material Nodes example level.