I have a 5kw mdkau so my experience may not directly apply. If the ECU does not see 120v +- in about 15 seconds it shuts the engine down. The generator applies a small voltage to the coils to start building the magnetic field and this builds to 120v pretty quickly. All of this goes through the brushes
on the rotor. If the brushes
are bad then the magnetic field cannot be established in the coils and 120v will not be generated. Brushes wear out and sometimes damage the contact rings when they do. I got 2300 hours on my first set of brushes but only 1050 on my second set because there was a small area on the contact ring that abraded the graphite brush. I replaced then replaced both pieces.
The circuit applying and controlling this voltage is called the field flash circuit and the only moving parts
(hence likeliest to fail) are the contactor rings and the brushes. I would look at the brushes first. Since you have already replaced the ECU where all of the electronics
are located you have likely eliminated that as a problem. The remaining parts
are voltage sensing coils which are a serious problem because they are embedded in the coils of the generator.
Like I said this information is for a 5kw mdkau and may not apply to your genset.