All you should have to do is replace the right side crank seal. You might be able to replace it with the engine still on bike, but I'm not sure, with mine I was able to. The seal could of failed from age or from worn out bearings that let the crank move around and wear it out. So wiggle the crank around to see if there it any play up or down. If your bike suddenly started to foul plugs when you noticed the drop in oil and the plugs used to be good looking but now are all covered and dripping with ooze, then its probably your seal. Ooor, the crank seal could be just fine and maybe you have a leak on the clutch cover or somewhere in that area where oil could leak out. Look for any oil dripping, especially from gasket areas. You may just need to get a new gasket.