Binnie:
As buickanddeere pointed out, the output voltage scales according to the speed if the excitation stays constant. So a 220 VAC 50 Hz generator would output about 264 VAC at 60 Hz.
You may be able to reduce the output voltage to 110 VAC with regulator adjustments. I've operated a 250 kW generator at relatively low excitation levels without seeing any problems, but that was a modern machine with a modern regulator. I would say try it and see, I can't see how you would hurt the generator by doing this.
At the same time, bear in mind that that by lowering the voltage this much you will be reducing the power rating of generator by approximately half.
Best regards,
Andy Hall