I'm in the process of removing the old insulation on my generator box. I removed the two sides and the top with relatively no problem. When I got to the back side it was a little different. Under the foam insulation is a felt like board. As far as I can see there is no sheet metal like the rest.
I'm not sure how to stick the new insulation on that type of material.
Ours has a plywood bulkhead between the generator and the propane/diesel tanks on the curb side of the coach. I scraped ours and will compress R-19 with backerboard fastened with standoffs and screws this fall. This is what I did in the engine compartment. Once the plywood is cleaned off, you could use staples and glue to fasten most any kind of insulation to it.
Pierce
We kept all the original insulation and covered it with double foil sided new insulation, mostly held in place with spray glue. Our original was premium with a lead layer for sound proofing.