There are two types of spray on "oven cleaner", one is for steel and the other is for aluminium saucepans. You have to read the labels to make certain it's suitable for aluminium because the one for steel will turn aluminium a very dark grey.

For whatever reason the aluminium safe spray cans were taken off the shelves some years ago here in Australia but they might still be available in other countries. When we could still get them it was fantastic. Spray it all over, wait about 30 seconds, give a light rub with a toothbrush then rinse with water. Job done. Actually I'd do it in two goes, one at the top of the engine then after that was rinsed do the bottom so you had somewhere to hold the engine. You don't want to get it on your skin because it burns quite badly (and is probably the reason it got taken off the shelves). Wear rubber gloves to be on the safe side.