Yes any brushed permanent magnet DC motor will work in reverse as a generator. It works just as well both ways. If you have a 500W motor then you can generate 500W with it. You can actually make a really good brake by hooking the motor terminals together thus shorting the generator. In practical use you would want to use a high power resistor. Not sure how much current you are running but you could use a relay to make a push button disconnect. When you push the button it opens one of the wires to the motor. You might also be able to put a diode inline with the motor to block the reverse current. It would need to be a seriously beefy diode though.
Sorry I have been busy and not online. That diode will survive as long as you can heat sink it. Looking at the design it is designed to be screwed into a panel or something metal that will heat sink it. I searched for a data sheet on it and found nothing. If it is a regular common diode then the voltage drop is 0.6V to 0.7V. So at 30A you are dissipating 21W. It will be red hot without a major heatsink. If it is a schottkey diode then the voltage drop is half of that.
A schottkey diode is about the lowest voltage drop diode around. The better way to do it would be to use a FET but that would require a control circuit. Yes you could tie a fan in parallel with the motor and when coasting either with or without the diode it generated voltage would run the fan. Keep in mind that when coasting with the motor terminals open, no current is flowing through the motor. Voltage is being generated but the circuit is open. As soon as you have anything that loads the generator current will flow and the power generated will be taken from the kinetic energy of the scooter moving. In other words when you load the motor being used as a generator it will be harder to turn and you will slow down. Also the type of fan will be important because if it is tied in parallel with the motor then it will experience a wide variety of voltage to operate on.