People who crockpot engines sometimes report a change in the colour of the crankcase Aluminium afterwards.
Locally a product known as . CLR Grease and Oil remover is highly regarded as something that cleans crankcases without altering them.
The product seems to be similar to CLR Grease Magnet as sold in the US.
If the cylinder is out of the crankcase you may find it easier to tap the Contra down the cylinder using a short piece of wooden dowel and a light hammer.
It should fall out once it reaches below the exhausts. It'll make it easier to clean that way. Otherwise I've found that tapping the contra up and down the cylinder a few times using the wooden dowel works well in freeing them from Carbon deposits.