Active ingredient is the Hydrochloric Acid
First clean out the tank of loose or large rust particles with a high pressure washer
The rust in the tank
Filled up the tank with 5L of toilet bowl cleaner and topped it up to the max with water and let it soak overnight.
First attempt failed as the toilet bowl cleaner ate through the aluminium tape I used to cover the petcock hole
2nd attempt, used a piece of metal and RTV'd it to the tank, this time it held up for the whole day
Next day, rust is gone! Well, mostly, but a huge difference compared to before
Finally I sprayed it with tons of WD-40, then flushed the tank with some fuel twice, then filled the tank to the brim with fresh fuel at the pumps.
Also, rebuilt my petcock from a repair kit bought from the UK online.
Also, changed my leaking fuel pump with questionable operation since everytime when the fuel gets low enough, the carbs doesn't seem to be getting any fuel. Changed to a Polaris china unit which costed me RM16 or USD4!
So far it had been a few days and inside the tank still looks rust-free, and the petcock overhaul and china fuel pump works well!