With out much more information, my guess would be the voltage regulator has bit the dust. Yes it probably is repair about. If the voltage has not spread beyond the voltage regulator it should be just a mater of removing a couple of wires, replacing the regulatory, and reconnecting it.

While it is a simple operation, exactly what has to be done and the parts needed will depend on the brand and model of your generator.

Contact the store you bought it from or do a search on the brand model and serial number of the generator. You probably can find the parts on line. You may also find the manual if you have lost yours.

Update; I did order an Automatic Voltage regulator and it did appear to solve the problem. FYI the regulator instructions said to set engine speed to create 62-62.5 Hz and I did so. That created 116 volts. BTW: before I replaced the regulator, I fired the generator up one last time to reset the speed to generate ~143 volts. However, this time the regulator was completely out of control with voltage all over the place. Fortunately after apparent successful replacement, I also found a country repair shop that works on portable gensets like mine (the more known shops only work on drive behind size portables). They kept it for a couple days (they were supposedly busy) and checked it out, told me it was fine and didn't charge me!

I sure never realized or heard of a generator running high before. Sounds like it could happen to others and there is no way to know in advance.