You should be able to download the OS from Lenovo. You will, however, have to be able to get into download mode. I am not sure how it works on that device, but you can try this...
1. Turn the phone off and then remove the battery for 5 to 10 seconds.
2. Re-insert the battery.
3. Now press and hold "Volume Down", "Home" and "Power" buttons, all at the same time.
4. When the ‘Warning!!’ message appears, release all the buttons.
5. Next press "Volume Up" button to confirm that you want to enter Download Mode.
6. Your device should now show a green Android with the words ‘Downloading… Do not turn off target’ below it.
This should allow you to download the OS from Lenovo. If it works. As I said, I am not sure it will work on your...