I replaced the insulation on my engine box and it helped a lot with the noise.  One caveat though is to take care of any other issues that could lead to excess vibration such as loose belt or engine brackets or motor mounts etc.

I think it took more than one kit, particularly for the fasteners and tape.  I epoxied the fasteners onto the box rather than use any sort of regular adhesive.  

The one downside is that the box is a little bit heavier than it was before, but that was a reasonable trade off for us.

I bought the stuff at Westmarine but it looks like similar kits are available at Defender & other marine supply places.  Something like this.  I don't remember whether I got the 1/2" or the 1", but I do recall that I agonized over whether the thicker would fit or not and did some "measuring" with the cover in place by peaking through the back side once I had removed the old stuff.  The 1/2" is probably the safe bet.

Rich, I checked the decibel level outside the box with the original material and then after I installed the two layers of first dark grey foam, then aluminum backed foam and found no real improvement, but the old stuff was falling off so it had to be done.  That was about 8 years ago.  While the corners that get squished regularly have been compressed and the aluminum has mariginally peeled off, in general it has held up.  See pics.  I got all the materials from Amazon, usually called automotive sound dampening material or somesuch.  

Other than the heat shielding added to the cover, are you doing anything else to address the remaining space open to engine, transmission, and exhaust noise and vibrations? Just curious since replacing the +30yr old heat shielding is on my agenda this winter.

This is essentially an infinite baffle similar to a large speaker enclosure (e.g., the trunk of a car and speakers in the rear deck. The cover seems like only a small part of the larger issue of noise and vibration abatement in the large, enclosed, very resonant cavity.