Here's how you can change font in KS Ex (see my signature):
Open the level with Level Editor Ex, press Ctrl+D and copy this line there:
Objects.SignText:SetCharacters(WorldPath.."font.png", 7, 13)
font.png is the Cyrillic font that you should put in your level folder.
I'd like to include your Cyrillic version of signs font and ego's clearer font with KS Ex.