I purchase the Bissel Brand of Oven Cleaner from BIG LOTS at a cost of 98 cents per can. For an engine, I spray a generous coat of oven cleaner, and then wrap the engine in plastic to keep the air from drying out the cleaner. I usually let it soak for a few hours, often overnight. Then use a pressure washer to blast off the loosened grease and grime. In many cases, it removes the paint as well.
For smaller parts, such as splash pans, air cleaners, etc. I place them in a BLACK plastic garbage bag and spray them generously. Fit in as many as the bag will hold, then seal it tightly. I place it outside in the direct sunlight so that the heat accelerates the cleaning action. After a few hours, I hit it with my high pressure water blaster. Again, most of the grease comes off along with the paint. I then go back and spray it again, sometimes three or four times until I have a part clean and shiny, ready for paint.
It is not magic, you still need to scrape off the heavy build up and let the oven cleaner work on the least amount of buildup as possible. It works great in tight corners where you cannot scrape the surface clean. I use it on just about everything. Won’t hurt structural copper or brass, but I wouldn’t use it on things like brass headlamps for example. It would likely damage the surface. Try it; I think you will like it.