Insulate the contact with floor and build a "dog house" with sound-deadening insulation inside. Ensure there is sufficient air circulation for cooling and "engine intake". Also a better muffler.
The easiest way to quite a load generator is to reduce places the metal vibrates. This could mean tightening bolts, putting in plastic or rubber washers, carpet underneath, etc.
buy a muffler for the same HP as your generator and weld it between the spark arrester and the muffler mounting flange. Put that back onto the generator and mount the muffler back too the motor.
