Holder 12' looks sportier than a sunfish.
Hobie Cat's answer to the Laser. You could possibly use a Laser or Sunfish daggerboard as a replacement. There is also a Yahoo group for the Holder 12' http://groups.yahoo.com/group/holder12/ where you might find other suggestions. I found them by doing a google search on: Holder 12' sailboat. If you are really ambitious you could fabricate (make) a new one if you still have the old one around to use as a model for the dimensions. Better yet, get someone to supervise the kids to make one themselves. I would use marine grade 3/4" ply for the basic shape and overcoat it with epoxy and cloth, then sand and paint.Good luck.