Hi if the motor is in good order and not 25 years old and dry it should be fine on 9v. I personally would never run them on 9v as it always kills the loco eventually. By killing it it either burns out the wound armature in the motor or it strips and breaks gears from too fast/too much power. You should get a replacement drive unit for your Gordon as he is one of the rarer locos that not that many of were produced.
In the end it's your toy and you can do what you like
In the end it's your toy and you can do what you like