Never ever use a lipo battery without protection against too low voltage or overloading. It might explode.
Most of the times, they can supply a large current.
Perhaps you need a larger battery, and some protection to stop the car if the voltage is too low (for example at 3.0V).
I think it is possible to run it directly from the battery, and have the Arduino keep an eye on the voltage. Only the sn754410ne needs 4.5V, so you might have to look for an other driver ic.
You could also buy a stronger switching voltage converter, like a buck boost converter. To make a stronger +5V.