According to PC Gamer a group of Russian mods known as World of Stalker are busy rebuilding S.T.A.L.K.E.R. using Crytek’s CryEngine 2.
CryEngine 2 was used in Crysis as well as Crysis Warhead, and despite its age, it is still considered one of the most visually impressive, albeit clunky, pieces of technology in gaming. Crytek has moved on to CryEngine 2’s successor, CryEngine 3 for Crysis 2.
The mod is far enough along for World of Stalker to be showing off impressive gameplay footage, which looks pretty much like what you would expect S.T.A.L.K.E.R. to look like running on the Crysis engine.
It remains to be seen what Crytek or GSC Gameworld has to say about the mod.