You may have other issues than just voltage and frequency.
All of the appliances
on the US boat will be either for 60 hz 120 or 240V and often no 240 appliances
So if you are able to convert your generator to 240V/50Hz, you also need to replace almost every AC electrical
item on the boat with a 240V/50Hz item. That could end up costing as much as a new generator.
FWIW almost all generator engines can be adjusted to run at the 1,500 or 3,000 rpm that 50Hz requires. And all can be wired to produce 240 V. The question is will the voltage regulator
work with the reduced rpms. Some types yes, others, maybe no. But I am sure that every generator manufacturer has the parts
to make it work.
Post the question on Trawlerforum. There are two generator pros that hang out there- Kevin and Ski.