Not a single speed fan, but my understanding is that when you strip all the junk off, just about any decent bike gets very light. Most of the late 80's and early 90's Centurions were better than just decent. Butted Tange tubing isn't junk.
+1
I used to ride a Centurion Sport DLX. Tange Infinity DB tubeset. It is still by far the smoothest riding bike I have yet to find.