- Pause the Animation in the Details Panel of the Body Component.
- Search Physics Constraint under the Add Component Drop-Down and make it a child of your new body Mesh.
- Make the Face a child of the Physics Constraint.
- Set the Physics Constraint Parent Socket to the Neck of the existing Body Mesh in the dropdown.
- Re-enable the animation to test it.
- You can check the Skeletal Mesh of the actor you want to be parenting to, this can help you to choose a good socket to parent your physics constraint to.