Delete the game four times, then change you username and password to something different. Go to game center and change the name, then delete the game again. Download it again and you should have all achievements and all things in the game.
To unlock the Formula 1 vehicle parts (such as the engine and rear spoiler), access the Stig Trail, building a car, viewing the Stig's lap and then racing him. As long as you get a Gold medal, you'll receive the parts.