In a microwavable bowl or saucepan, sprinkle the gelatin over the cold water; then allow the gelatin to soften for 5 minutes.
Heat until gelatin is completely dissolved.
Remove from heat.
Add fructose, sugar replacement, and margarine.
Stir to dissolve fructose and melt margarine.
Allow mixture to cool to a thick syrup.
Beat egg white to soft peaks.
Continue beating the egg white, and pour the gelatin mixture in a thin stream into it. (Mixture will not fluff up.) Place rice cereal in a large bowl.
Thoroughly fold egg white gelatin mixture into cereal.
Fold in walnuts.
Transfer to a lightly greased 13 x 9 in. (33 x 23 cm) cake pan.