Assuming? it's a fuel tank you're trying to replace?
The tank probably needs to be replaced with the same type that you're removing, if you can't fix the stock one.
Main reason being the fuel lines that won't line up with a generic version, not specific to your year/model bus.
The fuel lines might also be tender, depending on what they're made of.
Fortunately for you, the bus is new enough that replacement parts should be available still.
Unfortunately there might be other "downstream" issues that pop up when you buy a rusted bus.
Be prepared, and good luck!