I don't know about bus engines, but on a motorcycle, if the tank is rusty, ya gotta get a new tank! I would think the same would be true for any combustion engine gas tank.
I have heard there is an additive that will coat the tank walls and help minimize corrosion, but was warned it as a very temporary
measure, so never looked for it.
Good luck with your tank. Bites to have a bus and not be able to make it GO (I'm in the same boat now).