I agree with the other boys but depends on if you are going to repaint your engine???

I have often used oven cleaner but you need one that has a lower caustic percentage as cheaper brands have a high %
it cleans up metal amazingly you spray and leave on for a bit scrub and hose off (magic)haha
but use of the cheaper brands can cause some harm to the metal especialy if you are doing internals..

But i would go for the hot steam there are some companys that have really good ones that you can hire and ones you can add
chemical clensers too...that arn't harmful to you or your engine...