I much prefer the electrolysis derust method for the tank. Acid removes both rust and some of the base metal. Electrolysis just removes the rust. Both rust and acid can etch holes into steel.
I speculate the white stuff is Zinc oxide. The carb base metal likely has zinc alloyed in it. Many gas additives have an alcohol base. Alcohol will draw water out of the atmosphere and combine with other air impurities. Ethanol can be oxidized to acetaldehyde and further oxidized to acetic acid, depending on the reagents and conditions.
Acetic acid is mildly corrosive to metals including iron, magnesium, and zinc, forming hydrogen gas and salts called acetates.
I'm an amateur chemist at best.
For sure, you need to remove the white stuff to see how much base metal of the carb bodies has converted. You may also need to rod out the air jet passages as well and the pilot circuit pathways in the carb bodies.
The motor uses different alloys than the carbs do. So, unless the engine was submerged in water, it is probably okay. I don't think the carbs internals necessarily reflect what the motor insides look like, especially if it turns over. But, if the bike was a flood victim, who knows?
Cheers,
