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I stand corrected, there is One available in the US, and it’s $400 and in worse interior shape than mine is. 

The inside looks to be full of fuel varnish.  You need to get that stuff out before going forward.  I recommend fashioning plugs for all the tank openings, then dumping in some carb dip or similar.  Shake, rotate, soak, through all the various angles possible until you get all the varnish out.  Get a mirror on a stick, and a flashlight, so you can look inside the tank, all over, to make sure the varnish is out before going to the derust step.  After the phosphate soak, rinse with clean water, followed very quickly with some isopropyl alcohol.  A little light flash rust will form, that that won't hurt anything since you will be sealing anyway.  

So Nessism, sealing the tank will help the gas to stay inside, how about the outside? Should I try some fuel tank repair epoxy to cover the outside or just remove the emblem mount and Body filler straight to the surface? 
It also looks like the Amazon price and website price are the same. $18 to ship and around $60 for the product. 

If you soak the tank in phosphate sauce like suggested, that will derust behind the emblem bar so you don't need to worry about any more rust there.  You can just bondo the deep pits and paint the tank.  No need to remove the emblem bars unless you don't want emblems.

How long should the soak be for? Could the soak possibly mess up the body filler that has already been applied?

I use metal rescue and soak for 24 hrs minimum.

At $20-30 a gallon, does this stuff get diluted at all?

A word of caution...

Going back 10 years or so, I bought a KZ900 - it still had the original bodywork - in decent visible condition.  The inside of the tank was awful - so I cleared all the rust with muriatic acid - and coated the inside of the tank.  The tank was snug fitting on the new front dampers - so I pushed hard from the rear - and the tank top rippled...  the rust had destroyed the structure of the tank.  

wrote:

A word of caution...

Going back 10 years or so, I bought a KZ900 - it still had the original bodywork - in decent visible condition.  The inside of the tank was awful - so I cleared all the rust with muriatic acid - and coated the inside of the tank.  The tank was snug fitting on the new front dampers - so I pushed hard from the rear - and the tank top rippled...  the rust had destroyed the structure of the tank.  

YOU destroyed it Jeff!  The rust was probably holding it together!

Metal Rescue & Evaporust are great stuff, only expensive.  The phosphate stuff is what industry uses to derust steel.  It's safe and doesn't need to be neutralized.  Muratic acid (same as hydrochloric acid) is nasty stuff.  Very harsh and needs to be neutralized.  Phosphoric acid can be reduced, Metal Rescue and Evaporust need to be used full strength.

Nessism, the Kleenstrip metal prep is phosphoric acid correct? Thus meaning it can be diluted. Do you know with what so I can pick it all up at once? 

wrote:

Nessism, the Kleenstrip metal prep is phosphoric acid correct? Thus meaning it can be diluted. Do you know with what so I can pick it all up at once? 

Tap water, my friend.