The only correct fix is to have a reputable radiator shop cut a hole in the top find the loose piece, repair or remove it, and re-weld the hole up
I would term that action: "the incorrect fork-up" ..
Many's the tank that has been destroyed by such actions.
These fuel tanks were constructed of sheet-metal .8 mm (about 1/32") and terne plated on both sides for protection from corrosion. The black paint on the exterior is there simply for aesthetic reason.
Terneplate, steel sheet with a coating of terne metal, an alloy of lead and tin applied by dipping the steel in molten metal. The alloy has a dull appearance resulting from the high lead content. The composition of terne metal ranges from 50–50 mixtures of lead and tin to as low as 12 percent tin and 88 percent lead. The tin serves to wet the steel, making possible the union of lead and iron, which would otherwise not alloy. Terneplate is made by a process similar to galvanizing or tinplating—i.e., by dipping the sheets into a series of heated baths, the first of a zinc chloride flux, followed by the molten terne metal, and finally one of palm oil. Terneplate has the strength and formability of steel and the noncorrosive surface and solderability of terne metal. While it is still used for roofing, gutters and downspouts, and casket linings and in the manufacture of gasoline tanks for automobiles, oil cans, and containers for paints, solvents, resins, and so on, it has largely been replaced by other, more durable steel products that are easier to manufacture.
Welding on them is difficult and it destroys the solder coating but there are techniques that can be employed to open the tank, clean it internally and return it to service.
If unmolested, most can be repaired however it is a dirty, labor intensive job that I've done many times but do not relish tackling.