I did this engine detail a few years back on my 1991 Civic Si with 105,000 orig km
I used WD-40 and a paintbrush and 4 microfiber towels.
Works amazing on 20 year old buildup of dirt and rustproofing. No joke.
Car had been sitting for 10+ years.
I basically spent 4 afternoons/evenings total on it. (about 20 hours total)
STEP BY STEP:
1. Go to the automotive store, buy 2 cans of WD-40 , 4 microfiber towels, paintbrush.
2. Open Hood.
3. remove battery, air filter box and arm, fuse box cover, wiper motor, map sensors assembly, coolant overflow tank, move away side wires and tubes on shock towers.
4. Start attacking the dirtiest parts with WD-40.
5. Spray and brush/agitate the grime/dirt
6. Wipe off dirt with towel.
7. Once all the bay is clean, you can buff and polish the paint around the bay and dress the rubber hoses with rubber and vynil dressing.
Keep doing this until all dirt is removed. It takes a while, but well worth it.
WD-40 wont remove the paint.
Note: As you will notice, some parts were replaced/restored as well.