Runabout
I consider myself a master at patching things that should not be fixed and I love a challenge. Most normal sane people would throw that tank away and buy a used one, but not me. What I would try is to make two gaskets that match the pattern of the fuel shut off valve, then I would cut a piece of sheet metal to the exact pattern of that gasket, I would weld two 6mm bolts in the sheet metal. ( if they use a 10mm wrench the threads are 6mm-1.0 thread pitch) I would place the sheet metal and one gasket inside the tank after preparing the tank, then place one gasket and the fuel valve on the outside of the tank and using two 6mm nuts ( again a 10 mm wrench is needed)tighten and test for leaks. You may have to modify the instructions to suit the situation, but I think you will understand the theory that I am using.
I have patched much more complex things than this, but the double gasket on the plastic tank has always worked great every time I have used it.
Hope this helps. John