I'm no expert in racing, however the Flatwater race kayak I've seen are reeeaaallly narrow with a horn or similar projection above the waterline, behind the paddler which meets the required width. very round bottom, long waterline lenght and undoubtedly quite unstable.

Most of the boats discussed on this board are recreational sea kayaks, a diferent breed entirely...but you should beable to build a credible boat which would be fairly competitive in an ameture race.

I do not know of any published plans, but perhaps someone involved with college or Olympic kayaking could help.