Cleaning the inside of the engine before renovation

[Niklas.L]

I have been driving my Audi 100 2.3 quattro -92 for just under 20.000km now with it leaking just about every fluid there is. But now I have finally decided to go ahead and change the head gasket along with other necessary gaskets. I am also going to remove the oil pan and clean it along with the oil pump. Since I haven't changed oil in all this time, I have only refilled, I think the engine is quite greasy and grimy inside. So I want to clean the inside of the engine as best as I can before taking it apart. Both the oil system and cooling system.
What do you think is the best way to make it as clean as possible on the inside?

Regards

Niklas