Retarget Mecanim animation for bird

I purchased an animated bird from the asset store. I want to adjust the model slightly - just the mesh. I did this in Cinema 4d and it all works properly there. The problem is that when I save it back to Unity - none of the animations are working.

I have tried a bunch of things but nothing is working. The rig is the same - just the mesh has changed. Can someone explain the process for doing this?

May have to reassign the model to the rig, but also I have no clue if Cinema 3D supports bones, even then if the model wasn’t created in Cinema 4D then the rig probably didn’t transfer over.