Combine textures into one material

I have a model I made in Blender and textured in Painter. Both the model and painter files have all the mechanical parts with separate textures, so when I import into Unreal the mesh has a material slot for each part.

The problem is I want to reduce calls by having a single material, but the material slots were created by blender for the purpose of texturing separately, how do I have my model use a single material/material slot?? I have tried using the Texture Set combiner, but the textures do not map correctly to where they are supposed to go.

Is there an easy way to make models use a single material??

You could have a look at color id maps. You create them in Blender and than use it in Substance painter or another similar software