Ferroresonant CVT transformer is the term you are looking for, but note that they are frequency sensitive (And heavy), so if your generator cannot maintain a stable frequency you will get output voltage variation, they are the business for harmonic reduction however.
Names to conjure with "SOLA" and possibly the "AEL" group.
For slightly bigger machines, and if third harmonic from the load is a problem you can use a delta-star transformer, all the triplen then ends up as heat due to circulating current in the delta winding, common for stage power where large third harmonic loads from stage lighting are in play.
Personally for ~1kW class generation, if I had to use petrol (Yuck) I would just buy one of the Honda inverter sets (There are cheaper copies but you get what you pay for), and call it good, they really are exceptionally nice small machines, the EU1000i or EU2000i
Regards, Dan..