Insulation/Sound Deadner for Generator

Most DIY box stores carry a similiar material, or you could even switch to the white foam backed panels. All are very flexible. Another solution could be DynaMat. It is sold in auto parts stores as sound deadener for floorboards. Adhesive on one side and foil backed on the other. Very flexible and malleable.

Also check here for OEM stuff:

Engine room Marine Generator Soundproofing noise attenuation Gas/Diesel

Dynamat is simply a high mass material. While that does attenuate sound, I don't believe Dynamat is fire resistant and I would not use it in a marine environment. It is basically a heavy oil based material (tar) with glue on one side and foil on the other.

The other reason you don't want it is that you don't just want the sound to get out of the enclosure (by increasing the mass of the walls), you want to also reduce the amount of noise inside the enclosure by making sure the walls attenuate the noise and reduce reverbration. Adding wall mass helps, but if the walls inside the enclosure are hard, the sound inside the enclosure actually increses, so you don't get the improvement you thought you were going to get from an enclosure.

The link is good in that it has mass loaded vinyl materials that are a much better choice. They reduce the sound inside and outside the enclosure.

Dynamat has an approved temp rating of +300ºF, but it's up to the OP as to whether or not it's for him.

The temp rating isn't the issue, I've used Dynamat in a car for road noise reduction (plastic rear cargo floor area of a Corvette) and it is fine for that, and it does get hot back there because of the exhaust system being right under the floorboard.

My concern is what happens when it is exposed to fire or flame, and in particular on a vertical surface. It's basically a tar like asphalt material and if it ever lit up it would be very difficult to put out. A foam might burn, but not add a lot of heat because the mass isn't very high.

In my experience, boats and airplanes are places where conventional materials can become very dangerous, due to the implications of fire. I'd look for marine rated (and preferably fireproof) materials just for that reason alone.

Most generator manufacturers offer replacement panels through their parts distribution. Or you can purchase sheets from Soundown in FLL.