Tom,
I guess to escape potential liability I should state the same thing all others have stated, that being: "I'm not a builder, don't try this at home, the factory doesn't condone my activites, etc, etc, etc".
With all that being said, I personally think extending the vertical stabilizer and changing the rudder to a model IV rudder was the second best thing I have done to my airplane. The first being changing to Grove main gear with the tri gear modification. Adding the extra height of the vertical stabilizer and the model IV rudder got rid of adverse yaw in my plane. I can fly "hands free, feet free" and when I make turns they are clean and smooth (of course I still use the rudder, too).
You can purchase the model IV uncovered rudder from John McBean at Kitfox Aircraft. I think it was around $235 or so. I did not do my own work. I hired an A&P to do the work and I had him do so many things I couldn't tell you what he charged to do any one thing. To extend the vertical stabilizer he removed the fabric from the vertical, cut and removed the top of the existing vertical and welded additional aircraft tubing in place. The tubing can be purchased from Aircraft Spruce. I used a large piece of cardboard and traced around a model IV vertical from another Kitfox for the template. Seemed pretty simple and proved to be a great mod.
I did not make any changes to the elevators. I do plan to add gap seals.
And. . . I just added one more mod: A Lowrance 2000c GPS! Another great addition. See you at the 25th Anniversary Factory Fly-In!