Heat and sound require different insulations, if you want to reduce both you will need 2 layers. Outfits like Cotronics
sell ceramic papers and paint which will reduce the heat. Lead or asphalt sheets will reduce vibrations and are available from audio suppliers, B-quiet
and Parts Express
are a couple.
You also might try Cool and Quiet
, they make insulation for the hot rodders. Just make sure the insulation is rated for the heat and petroleum products it will come in contact with, a generator compartment is pretty nasty.