No need to worry! Firstly, no harm will come of the difference in frequency. Being an induction motor the fan will turn slightly slower, but so what? It certainly won't be noticeable in terms of performance. The heat rise is insignificant, especially for this sort of thing. As to voltage dropping devices, the easiest way is to use a capacitor. It will need to be rated at 400VAC even though it will appear to be dropping only 120V. Because the motor is inductive the voltage across the capacitor can actually be higher than expected. A motor start capacitor is ideal. The value will have to be found by experiment; at a guess start with around 4uF and go from there keeping an eye on motor voltage.
The other voltage dropping device I like using is an incandescent light bulb. I have not tried using a fluorescent light choke but see no reason why it too could not be used provided one of suitable inductance can be found. Resistors are another option but for the value required for this is not easily obtainable and needs to be well ventilated.
A cheap stepdown transformer is the other option, particularly if there's any doubt about the insulation of the fan at 230V.
I've been running Yankee stuff like this for years with no problems at all. In fact I fitted a 115V fan in series with a 2uF 400VAC capacitor into a fan heater, as a replacement, which has been pretty much in daily use for about 15 years.
