This problem comes up quite often. How to run an imported clock on a different mains frequency? There are several solutions. The commercially available frequency converter as mentioned above is the easiest answer - just plug and play - but it is expensive, costing considerably more than most electric clocks are worth. (Of course, if the clock is of sentimental value, you may consider the cost of the converter to be a price worth paying.)
For a DIY approach that requires only a few parts costing a fraction of the price of a commercial converter, I found this:
http://www.romanblack.com/one_sec.htm
Near the end of the page, it describes a crystal-controlled inverter and a mains-locked 50Hz to 60Hz converting inverter exactly like the commercially available frequency converter from
electric-clocks.co.uk
I've attached the circuit diagram. The source code for the PIC chip is here:
http://www.romanblack.com/onesec/ZE_SineConverter.htm
This will follow your local mains frequency for good long-term accuracy.
There's also a crystal-controlled version which will be as accurate as a quartz clock. It can generate 50 or 60Hz depending on the crystal, so you could change it to use a British clock in the USA if you needed to. Here's the source code but no circuit diagram:
http://www.romanblack.com/onesec/ZE_SineInverter.htm