Mecanim is somewhat a problem if it was used for animation... I don't quite know if there's a chance to export it.
Up to now I've only dealt with kexframes existing on the original model and then setting up an animation state machine in Unity.

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I'm doing it the other way round currently

Converting A8 levels to Unity.
Since I found the old FBX exporters for MED and WED this is going somewhat well, but in 99% of all cases and tries the texture assignment is lost.
For models this is not so much of a problem, but levels have many different textures... ack...

And I finally found the rootcause of the animation export problem to FBX.
It's a friggin' 0..1 numbering issue.
This makes the wrong pose being treated as T-POSe and messing up all keyframes.
The solution is simple: duplicate the T-Pose (must be first frame) and then export... I could have needed that solution some years earlier when I gave up due to animations not being eported properly...