The best replacement for twin turbo is supercharger.

The best alternatives to twin turbo are rated as following:
★★★★★  supercharger
★★★★★  single
★★★  single turbo
★★★  turbo
★★  biturbo
★★  twin scroll
★★  compound turbo
★  procharger
★  supercharged
★  bi turbo
★  supercharged 370z
★  turbocharger
★  single turbo supra
★  supercharged g35
  naturally aspirated
  supercharger 350z
  kenne bell
  blower
  single supra
  supercharging

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Speaking about twin turbo, one probably shouldn't forget about supercharger.

Other search terms related to twin turbo and their ratings are:
★★★★★ supercharger
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