Ahh! I remember the day I had to detail my santro's engine bay when we put it up for sale. Honestly, probably the most difficult part to do. First of all, go to the petrol pump and have it blown with air to remove as much dirt as possible (this will make it easier). Then get hold of a few plastic shoppers and wrap the ECU and the throttle body. Get a tooth brush and a bucket full of water+shampoo. Start scrubbing everything 
Get a degreaser if possible. Apply the degreaser on part with oil,etc. Turn on the water. Don't put any pressure to it. Slow flowing water where ever you've put shampoo or degreaser. Let it dry. When I did it, I sprayed Armorall tire foam on to the plastic parts and it did wonders. Gave it the new factory finish. Also, be sure to cover the plugs area. You wouldn't want water seeping into it.
