How to configure Xorg to use downloaded Sis 771 driver on Guix?

[znavko]

Hello! I use Asus K50C where VGA Adapter is Sis 771/671 Intel. It works on 
resolution most 1024x768, and this is incorrect (should be 1366x768). As I know 
only sis driver can help here.

I've downloaded Sis driver here http://w3.sis.com for Linux and want to try to 
configure Xorg to use it.
I am looking to manual and cannot determine how to configure Xorg to use sis 
driver:

Data Type: xorg-configuration  drivers (default: '()) 

        This must be either the empty list, in which case Xorg chooses a 
graphics driver automatically, or a list of driver names that will be tried in 
this order—e.g., ("modesetting" "vesa").

Please, help me with my Asus K50C.
Where to put sis driver? If I need to describe new package for sis driver it is 
impossible for me this time. Can you help?