Changing the 50Hz to 60 Hz is a matter of adjusting the speed control to produce 60Hz. Once you have done that you will also notice a higher output. 110 will approach 120. Using a 240 to 240 secondary center tapped you will basically have the same set up as what is delivered in North America. The downside is that when you run appliances
on 120 rather than 240 the current
required to do the same work doubles and the wiring
to your recepticles may be too light. On the other hand many generators have tap outs that can be changed to get different voltages. So if you are measuring it , someone may have already changed it and not changed the tag.