The losses will probably be somewhat higher at 60Hz c.f. 50Hz - you may have to marginally derate the transformer if you intend running it near its rated capacity. Perhaps the manufacturer can advise you, if still in business.
Don't be afraid to try it. Test the voltage under load and not. Monitor it awhile under some load. If is not getting too hot, good.
It will probably work ok on either 50 or 60.
125KVA is quite a big transformer. I wonder what flat5 imagines by "a while" and "too hot", because my idea in this case is to monitor it for tens of hours under half the rated load, you really don´t want such a beast to suddenly start on fire after you install it somewhere.
The temperature is even more complex problem, is the transformer oil-cooled? If yes, you should try to measure the temperature of the oil. But I have no idea what is the RIGHT temperature for such transformer. You should try to get some more detailed specs from the manufacturer.