Bring a large pot of lightly salted water to a boil. Add the pasta and cook until al dente, 5 to 7 minutes. Drain, transfer to a large bowl, and cool.
Whisk the sugar, eggs, flour, salt, and reserved pineapple juice together in a saucepan until smooth. Cook and stir over medium heat until the mixture thickens. Pour over cooled pasta, and toss to coat evenly. Cover and refrigerate overnight, or at least 8 hours.
Stir the pineapple chunks and mandarin oranges into the pasta mixture. Mix in the coconut if desired. Stir in the frozen whipped topping until evenly blended. Chill until serving.
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