2006 yamaha yz450f. coolant in oil

Very concerned just purchased bike and found milky colored oil. Was just recently rebuilt and bored before I bought it with new radiator and lines. Just learning in and outs of this machine so any help much appreciated. ......

A leak down test will find the issue .

A leak down test will only lead you to the cause if it's related to a combustion chamber leak, like a head gasket.  It will not reveal a leak at the water pump feed passage that runs past the base gasket and crankcase cover.  On the other hand, the head gasket is a more likely suspect.  But a head gasket can also leak from the coolant passage to the crankcase or to an oil feed passage without showing much of a combustion leak, too.  Iffy deal, but probably a head gasket.

Awesome info thank you.. going to start tearing into this weekend. And I dont think this is a small leak either. Almost seems like a hose is hooked up in the wrong place from the amount of antifreeze like they are connected.

If I open the plug by shifter for the oil the radiator basically drained down through the bottom end

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