[QUOTE=Got2Moto;20931181 Do I absolutely have to drain the top oil tank or can I just drain the motor?[/QUOTE]
If I understand your question correctly, I assume that you just want to install the new drain valve, not change the oil while your installing the valve.
Just for installing new valve you will only need to allow the bottom case oil to drain out. Then install the drain valve. Once this is done run the engine for a few minutes and leave the bike overnight. If in the morning you have oil leaking on the ground you get the privilege of removing the valve and adding some white permagasket tank sealant to the treads and re-installing it again, do not run the engine and wait until the next morning then run the engine. If you still have an oil leak, next step is drain, clean brass threads, install a new viton o-ring (local auto parts store), 24 mm gasket, more sealant and repeat the process. Sealant needs 24 hours to set.
Once it stops leaking you will thank yourself for doing it. Of course it might take two or three day to complete this little task, but that's why there's Rum or Beer.
I did not use the paper gasket which came with the valve. It was worthless. The bottom of the engine case seems to be machined to accept the 24 mm copper gasket and it was a sloppy fit with the new valve.