How can I make the mini inferno running faster ?

I use 6 GP AA batteries with orginal ESC/Receiver combo and servo. It runs too slow.
I had tried to use 8 GP AA batteries. It ran a bit faster but still too slow.

What should I do ? (I'm looking for a cheap method and don't want to buy brushless motor)
- Use 9.6V RC battery + orginal motor (or modified motor)
- Use 7.4V Li-po battery + orginal motor

any other suggestions ?

既然你提高左電壓都吾係快我好多, 好明顯係MOTOR問題, 換過隻啦

any motor model recommended??