I don't have this set, so I can't provide a fully authoritative answer, but 9V is likely to be safe for these motors. The hub that drives them is definitely 9V, as it connects with the rest of the 9V system and runs on 6 AA batteries:
TLG typically overspecs their motors somewhat, so you could probably also get away with running the motor using 12 volts, but it might harm the life expectancy a bit. If you can pull out the actual motor, you might be able to find a model number to look up for a more authoritative answer.
I'd recommend starting with a lower voltage (perhaps 3V) and working your way up to make sure that the motors are functioning as expected.