You can chase the petcock mounting threads with a metric tap the same size (5mm) with a standard metric thread pitch then use new screws. Make sure that the seal surface on the tank is smooth. Damage left behind left by corrosion will not seal. Another leak maker is a weak cross spring. It's the spring that keeps the selector gasket "sandwich" tight. Also make sure that the aluminum housing is not too damaged. These old petcock are tricky to say the least.
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