How can I retarget a Biped or CAT Rig

When trying to retarget either a standard CAT or Biped rig to use the UE4 Mannequin skeleton, the results are broken due to the mismatch in bone orientations.

Since Biped/CAT’s setup was pre-authored for max users 10 years ago and we can’t change it easily (or safely), what’s a solution? You can see I lost the use of CAT’s Limbs and that opposite limbs on the UE4 guy are completely axis flipped. What you DO see below, however, is a CAT rig that works in the engine perfectly. I have to choose between being completely useless in my authoring app vs being useless in my final product. I choose violence.

It seems the retarget system takes into account scale, naming…etc but nothing if my pelvis bone has Y forward and Z left against the UE4 Pelvis which is Y backward and Z right. Basically 180 yaw.

All other bones extend from this so I get his entire upper half flipping around, which twists every skin weight and my character turns into Jack Skellington.

Is there an “Animation Relative” option? I mean one that changes something?

Something with a world space to component space translation between target and source?

Can we separate rotation from translation?

Thanks for any advice

EDIT: All the information I’ve found online about this points to retargeting in the other direction, which transfers Biped/CAT animations onto the UE4 skeleton. I haven’t tried that yet but I’m guessing it’ll work. It’s opposite what I want to do though, which is take my already CAT rigged character and throw something out of the like 500 marketplace animations at it