If the motor has never been apart some rebuilding might be needed. If you just want to take care of the immediate problem, the impeller seal sounds like it went bad, backing up coolant into the crankcase. Kind of difficult to diagnose on the computer, but that is a problem I've encountered on my dirtbikes. I'm not 100% percent sure if your bike requires the cases to be split. When money is tight I like to start with the simple fixes first.