In a bowl, whisk together mayonnaise, scallion whites, kimchi, ketchup, lime juice, sriracha and salt.
Place whole unpeeled potatoes in a large pot with enough salted water to cover by 1 inch. Bring to a boil over medium-high heat and cook until potatoes are just tender, 15 to 25 minutes depending upon size. Drain and cut potatoes into 1 1/2-inch chunks as soon as you can handle them.
Transfer hot potatoes to a large bowl and toss with 2/3 of the dressing. Let cool to room temperature, or refrigerate until ready to use. Just before serving, toss with remaining dressing.
Adjust seasoning if necessary. Garnish with scallion greens and more lime juice to taste.