If you want a clean and tidy engine bay, either steam clean it or go at it with a load of 'gunk' or a similar engine de-greaser. depending on how handy you are, take off bits to get to the head/block and clean them up as good as you can before painting/prepping for paint. the items you take off can be then cleaned easily and put back. things like rocker covers are easy to remove, clean and spray up and can make a big difference. you can buy various products to make the plastics look good again. similar to back to black... start there and the more confident you feel the more you can clean up/paint.
if you do steam clean, make sure you don't get you'r electrics wet and do it with the engine running to help soften the crap and make the engine dry off quicker!
