1. Combine first 4 ingredients in a medium bowl, stirring until sugar dissolves. Let stand 30 minutes; discard garlic.
2. Preheat grill to medium-high heat.
3. Peel prawns, leaving tails intact. Arrange prawns on a grill rack coated with cooking spray; grill 4 minutes or until done, turning once.
4. Cook noodles according to package directions. Combine noodles, green onions, and remaining ingredients in a large bowl. Drizzle with juice mixture, tossing to coat. Top with prawns.