Cleaning inside the drive shaft housing gear case

I have a Nissan NS8B outboard. I was finally able to remove the LU to service the water pump impeller and wanted to clean the inside of the UU and LU casing from oily dirt and other contaminants. Are there any restrictions to cleaning fluids? What works best to clean out the casing and with what method? Once clean is there something I need to do to preserve the inside? This motor has been opened for the first time for maintenance since it was bought in 1993. Amazingly the impeller looks pretty good although I am going to replace it anyway. Thanks for any help.

Do a complete wp kit, not just the impeller.
You can scrub out the housing with scotch brite rubber-banded to a broomstick, or for really aggressive removal, a long handle on a wire brush. Use lots of dish soap.