This video goes over the process of how to use FK Retargeting within the IK Retargeting Toolset. While adding Full Body IK is nice, it isn’t necessary in order to get a retargeted result quickly. Often times, when dealing with bipedal characters that have the same proportions, but different skeletons, you can use FK Retargeting to get the results that you need. Let’s take a look!
https://dev.epicgames.com/community/learning/tutorials/r0dd/unreal-engine-fortnite-fk-retargeting-for-bipeds