Replacing Mesh
When replacing any kind of mesh, one must keep in mind that there are two ways it can be used. They can either be static mesh, or skeletal mesh.
Static meshes are used by objects that don't move, like levels and its props, or do move but in a basic manner, like moving platforms, projectiles and rigidbodies (physics active objects).
By default, when you add in a static mesh into Unreal Engine Editor, it would either come with materials set to default white or none at all (usually seen as a gray grid texture). You may add and setup these materials however you like to appear in the game. Some materials are special that they have parameters which are modified at run-time or are usually not loaded by default and swapped in later by an in-game event. Similar to any other asset, before cooking just make sure the name of the file is the same as how you found it in Umodel Viewer.
Skeletal meshes are similar to static meshes until a certain point. Skeletal meshes come with skeletons bound to them that will deform them like skin in game.
In addition to replacing the models and its materials, they also often are besides a Skeleton and Physics asset. In order for the mod to work properly, you must have these assets generated in Unreal Editor. Again, make sure they are named the same as the files found in Umodel Viewer.
If you want to have the skeletal mesh to be animated correctly, after the cooking is done delete the skeleton and physics asset. This will help make the animations work in the end when you try testing the mod later on.