When using slices of citrus fruits for garnishes, choose fruits with thin skins and not too much pith.
Wash fruit well and cut into 1/8-inch-thick slices. Remove seeds. In a small saucepan over medium heat, combine sugar and water. Bring to a boil and simmer until clear, about 5 minutes. Add only enough lemon slices to fit in one layer and simmer until transparent, 20 to 40 minutes. Transfer with a slotted spoon to a bowl and continue with remaining lemons; boil oranges, then kumquats, if desired, separately in same syrup. When all fruit has been candied, pour syrup over fruit and let cool. Refrigerate if not using immediately.
Heat oven to 350 degrees. Butter a 10-by-3-inch pan and set aside. Using an electric mixer fitted with the paddle attachment, cream butter and sugar until light-colored and fluffy. Beat in eggs one at a time.
Sift together flour, baking powder, and salt. Mix together juice and sour cream. Add flour mixture and sour-cream mixture alternately to batter, beginning and ending with flour. Add zest and beat well.
Pour batter into pan and smooth top with a spatula. Bake 1 1/2 hours, or until a toothpick inserted in the center comes out clean. Let cake cool for 10 minutes in pan, then turn out onto a wire rack, right side up, to cool completely.
In a small saucepan over low heat, heat marmalade and a few drops of water until liquid. To decorate, brush whole cake with melted marmalade and arrange drained candied citrus slices on top.