How di i convert a generator rated140v to 230v
You will need to adjust or modify the governor so the engine runs at 5/6 current speed (e.g. 1000/1500/3000 RPM instead of 1200/1800/3600). That is because 230V-land is also 50Hz.  

You will need to tune generator excitation so that it works correctly at this speed.

Then you will need to rejumper, rewire or rewind the generator for 240V instead of 120V.  This might be quite easy. Or prohibitive. Depending on the generator. 

Then you need to move the neutral-ground bonding so it is corner-grounded instead of center-grounded, except if you're using it at a construction site and not interlocking it with utility power, I wouldn't bother. Center-grounding is a virtue in that application; Britain even requires it.  

Finally change the receptacles from North American to the ones you find convenient.

Plan B: sell it on Craigslist to some American who brought over US tools without thinking about voltage, and buy the right thing.