Apparently, some kind of an animal possibly a squirrel or some other type of rodent got in there and died. Is there a way to flush out the bilge with enough water to get the carcass to float down to a more accessible place where you can get it out of there. You may be able to pitch the boat on it's boats stands and keel blocks, to where the water can run forward or aft. If you can find out what it is and remove it, the next step would be to get the boat level and use a good bilge cleaner or perhaps water mixed with baking soda and let it stand for a period of time. Also, there are companies that specialize in cleaning gasoline fumes out of boats. One of our club members experienced a gasoline leak in one of his two Aluminum tank on his 26' cabin cruiser this past summer and the company that cleaned it up did a superb job. Nothing can stink worse than gasoline except maybe a skunk. If you really need to cut in a deck plate, it would be wise to locate it in a place other than on the cabin sole itself, but close enough to where you can reach in and grab whatever is trapped inside the bilge. Good luck with this one.