Well the key to getting your engine clean is patience and a bunch of elbow grease.
If it's just the paint that you want to clean up here is what I would do.
- Buy some inexpensive microfibers or shop rags.
- Purchase a good quick detailing spray like Meguiars Final Detail or similar. If your paint is in really dirty condition use Simple Green Automotive.
- Either wet your cloth or spray the product directly on the painted surface. If it's really grimy you may need to use a Boars Hair Detailing Brush which is available at most Auto stores. The brush is soft enough that it won't scratch the paint but hard enough to get the grime off. If you started with simple green use either water or a quick detailer to wipe the surface again. Follow up with a good paint sealant wax like Meguiars #20 or Zaino. If you want to really polish the paint use a cleaner wax or a dedicated hand polish first and follow that with a wax.
Like I said, patience and elbow grease results in quality work.
Good luck and post some before/after pics if you can..