Motor shield leaves no pins accessible
According to tutorial, the shield uses almost all D pins. Free are pins 2 and 13. If servo is not used the pins 9 and 10 are free too. The pin 2 has the extra hole to solder a wire or a single pin header.

All analog pins are free and the shield enables to solder headers for them with additional header rows for 5V and ground for every A pin.

You can use analog pins as digital pins to connect the ultrasonic sensor.

Connection of I2C devices is possible on A4 and A5 pins.

The shield doesn't alow to stack a next shield, because it heats up.