Hi, I had the same problem with my Sherco fuel tank. After a couple of attempts with J B Weld which didn’t last very long, I cleaned the tank area around the plastic block and tank and then keyed the area by taking a hot soldering iron and stippled the same area of the tank outlet.
Take care not to puncture the tank. I used plastic padding leak fix tank repair which I built up around the joint,
you have to work quick as it doesn’t take long to go off.
This repair has been successful to date. Good luck.