My bet is head gasket since it gets into the crankcase so fast. A simple compression test will tell you if it is the head gasket.
If the compression is still good the odds favor the water pump being bad.
Now with that said, IIRC (I don't have the service manual handy) pulling the water pump to check it and/or replace the seals isn't too difficult (and in any event easier than changing the head gasket.)
Of course you do realize you must not ride the bike till this is fixed.....
I have come to terms with the possibility of dying while riding, life has a 100% death rate. I would rather live having fun and end up dead in a motorcycle crash than never do anything fun because it's dangerous and I might get killed. No one makes it out alive anyway. - From a friend on another forum, used with his permission.