Steam sweet potato in a steamer rack, covered, over simmering water until just tender, 7 to 10 minutes. Cool to room temperature.
Meanwhile, cook choclo (if using) in a medium pot of well-salted boiling water, stirring occasionally, until heated through, about 3 minutes. Transfer with a slotted spoon to a bowl, reserving liquid in pot, and cool to room temperature.
Return choclo-cooking water to a boil (or use 4 cups fresh water if not using choclo). Add scallops and poach at a bare simmer, stirring occasionally, until barely cooked through, about 1 minute. Drain well and cool to room temperature.
Halve onion lengthwise, then very thinly slice using slicer. Soak in a bowl of cold water 5 minutes, then drain well and pat dry.
Stir together lime juice, pisco, chile paste, and 1/4 teaspoon salt in a large bowl, then gently stir in sweet potato, choclo, scallops, onion, bell pepper, cilantro, and salt to taste.
Serve ceviche on small plates, drizzled with any remaining lime juice mixture from bowl.
What to drink:Concha y Toro Terrunyo Casablanca Valley Sauvignon Blanc '07
Cooks' notes: Sweet potato can be steamed 1 day ahead and chilled.