Do as mentioned above, in addition there is Allen head screws on the block toward the back you can remove to completely flush block, but that's extreme. The brown gunk u see is probably leak sealant. Once you flush the system you will discover leaks around the horrific water pump design. I would drain and flush raidiator. Fill with water drive flush, install
antifreeze. Distilled water in this case is like marking a piece of wood with a 5mm pencil and then cutting with an axe. Use at minimuim 180f thermostat.