check this:
http://mosthostla.com/gamedev/bonebreaker/
For a 20 sec. split of the mannequin, export any animation of the mannequin or your character rigged/re-targeted to the epic skeleton with LODs.
In each LOD, select it, go into edit mode, and press P - click separate by loose parts.
The mannequin might split (should) lengthwise across the middle.
You just click the 2 segments and CTRL J to join them back.
Be aware that there are a few pieces on the mesh like the inter-costal or the neck that are comprised of many parts rather then just 2. It takes a min to sorer them back to the correct section.
Once done you can save this as a master file, and start exporting.
Don’t touch the bones, eliminate just anything but the body part you need.
Export as FBX and import back into UE4 - associate with the existing mannequin skeleton/epic skeleton.
that’ll get you started - you can even separate the fingers if you need. lots of meshes = more sync required.