Need some advice here. I have a JD 1520 gas. Accidentally left straight water in cooling system last winter. Started it up, immediately noticed knocking, shut it down. Found oil looked like chocolate milk. Drained oil and dropped oil pan. Looking up into bottom of engine I don't make out any cracks in the block or problems with the bottom of the sleeves. How do I figure out how the water is getting in the crankcase? I'm thinking head gasket pushed out, cracked head, sleeve gasket(s) damaged, or cracked block. How do I isolate the problem most easily? Thanks in advance for suggestions.