The current engine as it stands is the only thing capable of supporting a game like esf. The Unreal Engine is versatile, and the new UDk4 supposedly has tools which make many aspects of development "EASIER" but at this point in development it would more than likely add several more years onto the development cycle.
ESF is a heavily modified engine overly that requires the Half-Life file resources to operate in a very specific way. And if I'm not mistaken, they would need the source code for the engine(which could take years learn the in's and out's of) to even attempt to make ESF work in UDK and I'm not sure if that has been released to the public.