Is there a nose hook modification for a standard cirrus

In general the cg hook is not an issue on S-H gliders if the glider is correctly set-up. The important things that need to be checked and done:
1. Replace a tail skid with a tail wheel.
2. Make sure the tail wheel is rubber and not hard plastic like some of the early models.
3. The tail wheel needs to be aligned with the glider so that it tracks straight. This can be tested by holding the wings level and pushing the glider on a flat surface for several hundred feet. No deviation should be found..

If all of the above have been done the glider should track straight on normal take-off. You can also hold back on the stick during the initial few seconds of roll to help hold a straight line in cross-winds until more rudder athority is gained and the tail is raised.