Migrating animations from different bone structures?

I have a number of different animations from Unity that I’m trying to bring over to 1 skeleton in UE4. These animations are applied to different bone structures that don’t line up UE4’s skeleton / model.

Do I need to retarget bones using Maya or Motion Builder? If so what’s the best way to about doing this, should I constrain the bones with parenting one by one trying to match them up as close as possible? Or something different?

Constraining the old bones to the UE4 bones has only lead to issues as the number of bones differ.

Without wasting a lot of time I’m posting this here to see if anyone can help this process for me and others.

Tl:dr: What’s the easiest / fastest way to go about migrating animations on different bone structures to another bone structure for use in UE4?

Any advice is greatly appreciated!