Retarget Metahuman face animation to an external custom skeletal mesh with different bones.

The face animation done with ARKit doesnt need anything special.

If noth rigs are correctly rigged, all you need to do is have an animation with rhe abimated curves.

Unlike bones, curves are freestanding. You can have any curve on any skeletal mesh.
Thats why copy pasting the curves would work fine to move the animations between assets.