I recently had a conversation with some guy on macrumors who's working on an Amiga port of the Xash3D engine.

Here's his work:
https://github.com/AmigaPorts/xash3d-fwgs/tree/feature/amiga-support

I have been able to compile his code on PPC OS X but I was unsuccessful in getting the game to run unfortunately.

I was wondering if anyone here could give a MorphOS build a try.