that stuff you are considering will attack aluminium alloy. You might not see it and it could also loiter in the metal's surface for a while.. but anything that causes oxidation or discolouring is affecting the metal.. Okay, that doesn't matter on the outside of a crankcase... but you could possibly weaken or make brittle any of the hardened or smooth working surfaces inside the motor, if you're not very careful..
Forget anything to do with water, Coke, carbonate or bicarbonate, witches brew, tea bags or any other nonsense. If you want to dissolve anything you need a solvent.. Water is not
a good solvent for oil unless it's super-heated and has detergent in it. Forget all about it.
Use simple, boring old kerosene
(aka paraffin or aviation spirit). It doesn't affect metal at all (even after full continuous immersion) and will dissolve all gum and varnish. Kerosene is the active ingredient in any fuel system and oil-flushing product, so if any is left behind, it will get absorbed by the oil and not do any harm whatsoever..
If you have a death wish you can mix a bit of Toluene in there with the kersone and then heat the mixture (though not over a naked flame..
) to give you more strength.
Kerosene is also cheaper than all those other solutions.. and when you've finished with it, you can dispose of it with any old engine oil (down at the recycling plant) rather than tipping it in the drains..
Inside the cases will come up gleaming (invest in an old toothbrush to help shift the gum and varnish) but the outside cases will be corroded and nasty. The photo shows cases that were blasted clean (on the outside.. never blast the inside).