If your previous purchases are not showing up in your game you can try the following steps when purchased with your Google Wallet:
Firstly, make sure you are logged into the same Play Store account that you made the purchases with. If they still haven't been restored automatically, force close (the game should not be running at all at this time) the app and try clearing the data for Mini Motor Racing from your device, then restart the app. You can do this by going into Settings -> Applications -> Mini Motor Racing -> Clear data button.
Please note: this will also delete your career cash and progress.