84 vf750f, issues after sitting over winter

well long time no post, its been a while since ive been on here but my 84 750 has a serious issue. so slowly im selling this old girl to my brother because not only is he family, but its probably the one thing he is willing to give up his vices for. Anyway, i cleaned the carbs on it like i do every year before i let him start riding it. i went to check the oil and what i saw... milkshake... according to my brother last summer he changed the oil on it 2 days before we put it up because the cheep chain he bought took a crap. so after a week of riding after oil change and a winter of sitting and we have a dark, milkshake, antifreeze filled crankcase... how could this happen? im hoping that the water pump shaft seal went (easiest fix) otherwise im looking at a head gasket change and possible tear down of the motor. any ideas?