Got into the same problem with diesel all over the place when the return line let loose and the daughter was using the car for the week.
I put a half gallon of Simple Green into a garden sprayer and soaked up the engine and hood pad. Engine was cold and I let it work for a half hour and sprayed the whole mess off with a hose. Engine came out bright and clean, diesel smell was gone and a mint odor wafted throught the car for the next week.
I read here where the Orange cleaner works even better. Use the same method. Both are sold by the gallon at Home Depot, and most of the bigger hardware stores for a reasonable amount.
I found this works well and have been using the same on both of the old diesels. Keeps under hood clean. Muck easier to work on.