To design a circuit that controls the travel time of a 7.6V vehicle, a pulse-width modulated (PWM) speed controller can be used, which adjusts motor speed but does not inherently limit travel duration. The 555 timer circuit mentioned can function at 7.6V, but it needs to be configured correctly; an astable multivibrator controls speed, while a monostable multivibrator can manage duration. Combining both configurations may effectively achieve the desired control over travel time and distance. The practicality of modifying the existing circuit depends on the specific schematic details. Proper integration of these components is essential for achieving the intended functionality.