Finding a single speed kit for an old campagnolo from 1997 with screw on freehub
That doesn't look like a screw on freewheel hub, that's an 8 speed freehub designed for a slide on cassette. I have the same one. Its the older design called Exa drive. When Campagnolo went to 9 speed the shape of the spines were changed and made deeper. This is why your conversions kit doesnt work , as its designed for the newer style freehubs. See the page below 

http://branfordbike.com/articles/cassettes-and-cogs-pg60.htm

Unfortunately you cannot just swap the freehub body for a newer 9/10/11 speed one as the axles are different sizes. You either need to buy a newer style campagnolo hub, or you can take a cog from a Campagnolo 8 speed cassette and use spacers either side. You may find the chain jumps of the cassette cog though as the teeth are not as deep as a dedicated single speed cog. 

If you're buyng a new hub/wheel for single speed you may even want to buy a Shimano one as they are cheaper and single speed cogs are more widely available in a range of sizes.