If the frame is steel spread the rear stays slightly - 2.5mm each side. You can also look up 'cold setting' if you want. There is a potential for the smallest cog on your new cassette/ wheel to rub on the stay but try it first if you can.
*here you go:http://sheldonbrown.com/frame-spacing.html
I have done this, I.E. using a blank of wood, it worked.
Nothing to loose its only a £30 bike, just messing about trying single speed. Just needs some rim tape for the new wheel and chain then I can give it a go.
I may have an issue with the U-Brake and the slightly narrower rim.