Picked up a project, 2002 YZ 250. They rode it without coolant and blew a nickel sized hole through the center of the piston. The guy I picked it up from said the radiator had a slow leak. I have no idea where. So far I've cleaned out the metal shavings from the crank case and am preparing to bore it. But when I drained the oil, it was the color of chocolate milk. Light brown, completely. I know this isn't normal. My dad said when he broke the engine on his chevrolet tahoe, water got into the engine causing the oil to turn light brown. I think the radiator somehow leaked into the oil? So far my plan is to replace all the gaskets... any ideas appreciated.

Always a possibility that the head gasket is faulty, or maybe the water pump seal has gone. Not familiar with bike so I cant be more specific than that.

Picked up a project, 2002 YZ 250. They rode it without coolant and blew a nickel sized hole through the center of the piston. The guy I picked it up from said the radiator had a slow leak. I have no idea where. So far I've cleaned out the metal shavings from the crank case and am preparing to bore it. But when I drained the oil, it was the color of chocolate milk. Light brown, completely. I know this isn't normal. My dad said when he broke the engine on his chevrolet tahoe, water got into the engine causing the oil to turn light brown. I think the radiator somehow leaked into the oil? So far my plan is to replace all the gaskets... any ideas appreciated.

If there are no obvious external leaks then the next on the list is the water pump shaft seal. As above.