I just replaced my hood pad, make sure to cover the engine bay with plastic or some kind of protection because the glue is PIA to remove if it drips on the engine bay... I used 3m from the dealer. it works well.
I used double tape to position the pads under the hood and sprayed the glue in small area at a time.(once the pad and the hood get stuck, you can't reposition it, at least with the 3m glue)
Regarding the engine, the last time I did this, I had problems with my car for a couple of days.hard starting, and rough idle, hesitation....in my opinion it's not worth it.
The way I clean my engine is that, I divide the bay into 4 areas and concentrate cleaning on each area with degreaser (on a small spray bottle) till it's clean. It is hard work but you know you won't have electrical problems. I stopped spraying water in my cars engine bay since they came out with the ECU. I clean the engine bay at least every couple of months just for maintenance. The first time is the hard part, the follow up is easy.
Others might not agree with me but that is my .02cents.