hah, that exactly what happened to me ML-toys, I had purchased a running Cannondale cannibal engine, and started building a kart frame around it, shortly after I was given a Complete cannondale cannibal, with a milked engine in it. I just this past weekend, finished it, and can give you much info on it if you would like.
The engines in these bikes had many factory service letters for them, and had alot of updates.
I found out the reason the engine got milked (oil and coolant mixed in the case) on the bike i was given, was the use of wrong coolant, the impeller for the waterpump is bolted onto the end of one of the cams, and the cover that sits over the impeller that makes the "water pump" seals on the side of the valve cover.....here is where the problem is, some of the valve covers are made of magensium, and if you use the wrong coolant, it will actually eat through the valve cover and into the engine mixing oil and coolant. wich is what happened to mine. For parts look up Black Widow Atv. Lemme know if you have any questions,I just did mine from the ground up. and I can point you to all the manuals,and addendums for them if youd like