Simple Urgent 3D Game Developer

The app is a racing game. I have source code of this game. The game is in Unity 3D format. I want to implements: 1) Change some graphics and Sounds. (I'll provide all designs and sounds to change) 2) Add 2 power ups, one that acts like a magnet that pulls in visible coins and timers for a certain period of time. The other one lets the user not lose time even if the car hits the other cars. 3) Coins collected can be used to buy any of the 3 power ups. Player can also buy additional coins through in-app purchase. 4) Facebook/Twitter integration. Thanks.