Beat eggs for 5 minutes. Add sugar. Stir in pumpkin and lemon juice. Stir in flour, baking powder, cinnamon, ginger, nutmeg, and salt. Fold into pumpkin. Spread in greased and floured 15x10x1" pan with waxed paper. Top with walnuts. Bake at 375 for 15 minutes. Turn out on to towel sprinkled with powdered sugar. Starting at narrow end, roll towel and cake together. Cool. Unroll. For filling, combine powdered sugar, cream cheese, butter, and vanilla; beat smooth. Spread over cake. Roll. Chill.