I have seen half life working for unreal engine, you loose the jabber mouthsand I think it breaks some scripted scenes but I believe that it is playable through. They hacked together something mostly for PSP, I never really got it moving as files were difficult to track down without joining several forums and gave up.
Here is a video of it in action and hopefully some links to follow.

https://www.youtube.com/watch?v=Dt_ODZMOwS0
https://www.youtube.com/watch?v=YA9km4aHjzI