Using 12V AC 50Hz Water Pump on 12V AC 60Hz US Power—Will It Work Safely?

My girlfriend purchases a small water fountain in Europe that I am trying to get working here in the US. The power supply provided with it does not support an input voltage of less than 230V AC. When using it with an American outlet I am seeing reduced power and the pump cannot move the water to the required height. I am leaning towards buying a 110V AC power adapter and wiring that to the provided pump. I do not want to replace the pump as it would be difficult to install a new one (wiring, waterproofing, etc).  A problem I see is that US power is 60Hz and the pump advertises that it needs 12V AC @ 50 Hz. Do you see any problems using 60Hz (and potentially giving it 20% more power than it wants)? If so, is there a cheap solution to convert 60Hz into 50Hz? Otherwise I'll have to replace the pump which will be much more tedious.

== Pump == 

  12V AV @ 50Hz, 2W. Capable of moving water @ 250L/H

== Supply == 

PRI: 230-240V AC ~50Hz

SEC: 12V ~200mA 2.4VA

This is a common problem, there is quite a lot of info on the web, try http://www.eng-tips.com/faqs.cfm?fid=1224 for a start.
Run it from a PWM DC supply to get the desired frequency if needs be,
cheers,
Richard