See my full post on how Steinberg provides a free converter for 1 month,
From my memory a .sng file is a Cubase Part. If I am wrong, I am sorry for wasting your time.
I believe that .sng file would convert to a modern CPR file using the "import Cubase part " feature under the file menu of Cubase Sx3 . If it loads then use save as and then you have converted to the modern .cpr format.