looking for off-the-shelf regulator for 5KW" off-grid" induction generator?

Typically below 12-15 kw most generators are rpm governed for voltage and frequency. Larger units have their own voltage and frequency controls that work to a tighter level. You do not mention what your trying to do. If you would describe the application maybe the group can make some suggestions. Depending on the application the voltage can vary +10%/-10% and have little effect on most equipment. Except for the vary sensitive.

I usually set my gen with a frequency meter and 50% load. Voltage at idle is 130 ish no load. When I start loading it down, the voltage drops to about