Personally I use MotionBuilder as retargeting is one of the tasks that it does by design but Maya has a sub set that I believe they call FBIK (Full Body IK) which also has the ability to retarget animation from an imported animation onto the changed or custom character rig. I’ve never used it so can not advise you of the process but the basic function is to characterize the two data set and transfer the matching animations from on rig to the other via the Control Rig.
A very basic overview of using FBIK to retarget animation in Maya.
For it to work the way it’s suppose to you need to characterize both character inputs/outputs.
Not sure why Epic made an animation tool for use in Maya as the FBIK is already an advanced animation tool that uses the same technology use in MotionBuilder.