Fuel tanks with heavy rust deposits inside can be cleaned with Acid you can purchase at a hardware store, about 5$ a gallon.  You can dilute the acid with water, I use it full strength. The fume are very noxious, you must do the cleaning in a very well ventilated area. (outside)
    Muratic acid will destroy aluminum very quickly, the fuel valve must be removed.
Add about 1 quart acid, and roll it around in the tank to coat the surfaces, you can leave it in for several hours if needed. Light rust will be removed in minutes.
    When the rust is loosened, flush out the tank with water. The clean surface will rust in minutes after you flush it out, as the acid cleans it well. Spray a WD40 type lube inside while getting out the last few drops of water to prevent new rust.
    I would not recommend using a liner inside the tank unless you have pinholes.  I have repaired leaks by cleaning of paint to the metal, and coating the outside of the tank with Devcon  5 minute epoxy. The fuel does not affect the epoxy.
     Be careful, wear safety glasses, read the label on the acid container.
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