Fuel contamination? Any idea

Pull the gas tank and drain/clean it. reinstall with new filler grommet. pull carburetor and clean it. draw fresh fuel through the system and change the fuel filter. Change the motor oil in your engine also. Any clues by smell/consistency to give you a preliminary guess as to what it is? Luckily the valve train is easily lubricated and bent push rods replaced. If your tank wasn't full of sediment before, I agree that vandalism must be what happened here.