i would think rewiring the motor would be useless -- as mentioned above, it's probably a sealed / potted unit that you can't open, much less work on.
on the other hand, i really doubt that the impeller or the windings would care if they were being driven at 60 hz instead of 50 hz; i would be surprised if the impeller is in fact different from those installed in systems sold for US standards.
as ZN says, the little up-converters are probably not intended for continuous use, but if enheim can't help you, you can probably get a small 2:1 transformer at radio shack that will do the job. unless you can find an Aussie-style outlet you'll have to directly wire the pump to the transformer in the box, but it should still be pretty simple and inexpensive