That is a brushless capacitively regulated generator. The uF rateing of the capacitor determines the output voltage. It basically charges from it's coil based on load, then discharges back thru the coil and induces a voltage in the rotor input windings. This voltage is rectified by diodes located on the rotor and fed to the rotating field windings which induces the output voltage in the main stator windings.
Carefully check the electrical connections at the capacitor. Also carefully clean the cap to make sure there is no path for current to bypass the cap which would reduce it's feedback to the rotor. The diodes that rectify the current to be fed into the field are located in on the rotor shaft. You can probably access them by removing the generator end plate/cover. You will need a volt/ohm meter to check these, and it is best done with at least one of the two leads of the diode disconnected to get a good reading. A diode can be thought of as an electrical check valve. When measured with an ohm meter, a diode should have infinite resistance(open, no current can flow) measured in one direction and near a short(full current flow) when the test leads are reversed(measured backwards). If this genset has the 50HZ optional connections, make sure the output windings and the capacitor connections are all configured for 60HZ operation. The manual shows what should be connected where. If the diodes check out, and all connections are good and capacitor is clean, recheck the voltage at 60HZ on the killowatt. If it is still low, try applying a little load. The capacitor is charged based on load current. That is how it can compensate for load increases, so a little load may increase your output voltage. Some loaded runtime may improve it's output also since it is, i assume a new generator. If it remains low, with good diodes and connections, the capacitor is most likley at fault. That is the most likley point of failure in this type of generator setup.
