A good hand full of old nus an bolts dropped in the tank shake for a little time to get the loose stuff off blow out with air line then tip in some Fertan or similar rust killer shake tank leave for a day tip in a bit of petrol good shake leave for a bit tip out petrol tanks ready to refit. get Fertan if you can it's the best
-but I have used this product with quite a bit of success http://www.evapo-rust.com/
-see here for another site (I am a member of) which has a couple good threads
http://forums.kz650.info/index.php?topic=5215.0
http://forums.kz650.info/index.php?topic=9032.15
Just remember to get some type of oil in the tank asap so it won't begin to rust again before you are able to install.
I can't find anything at the moment on electrolysis but I know that will work too. -You hook a battery charger up and fill tank with water and hook up positive side to a rod without touching the edges and then by moving the negative side around the outside of the tank you will get the job done. PM me if you think you might want to try this and I will look harder and help with the set up.