You can purchase many different LDO regulators that will do the job for you, but you might consider building your own from simple components. Here is a circuit that holds a 5V output during a 50 ms input voltage sag from 6V to 4.5V, with a 100 mA 50 ohm load. This simulates a voltage drop when the 6V DC motor starts. It uses an LED as a 2.5V reference, which may be stable enough for your purposes, or you could replace it with a more accurate reference, but the LED also provides a handy power-on indicator. The output voltage can be adjusted with the voltage divider R2 and R3.

You can purchase many different LDO regulators that will do the job for you, but you might consider building your own from simple components. Here is a circuit that holds a 5V output during a 50 ms input voltage sag from 6V to 4.5V, with a 100 mA 50 ohm load. This simulates a voltage drop when the 6V DC motor starts. It uses an LED as a 2.5V reference, which may be stable enough for your purposes, or you could replace it with a more accurate reference, but the LED also provides a handy power-on indicator. The output voltage can be adjusted with the voltage divider R2 and R3. This design can work to an input/output differential (droput voltage) as low as 300 mV at 100 mA.