Flushing and cleaning a tank

I would remove the tank, remove the fuel pump assembly and drain out all of the gas. Rinse the tank out with fresh 1/2 gallons of gas several times. Adding some fuel system cleaner (STP or GumOut) to the gas may be helpful. You may want to let one of the rinse cycles sit over night sloshing the gas around occasionally. Install a new fuel pump or test it. Install a new gas filter. The fuel lines to the engine may also be gunked up. I would suggest a method for cleaning this line but it is potentially dangerous. If the engine has not run in 8 years you should prepare the engine for this dry start up.

If the gas has turned to orange goop, I would replace it as you will have nothing but problems since you will never get the tank clean enough.

To follow up on what JonCG said about a dry start-up. First buy all new spark plugs then while they are out (label each wire so it goes back where it belongs) and get an oil squirt can and put a few drops.. NOT TOO MUCH of motor oil, light oil like 0W20 in each cylinder and rotate by hand several revolutions. You might as well plan on a new carb. Change oil. Coolant. Battery. Fuel pump.

I drop a bottle of rubbing alcohol in the tank to displace any water that has been put there by heating and cooling the liquid gas in there. Take the gas line loose at the carb and let it pump a little out via the pump and see if it looks clear or has little crusties in it. With only 5gal, filling it up and putting in the alcohol should keep the old gas good as diluted with new... run for a while. Also use real gas and not the corn gas to fill up. Depending on when the last gas was put in (meaning winter or summer time) will determine what is actually in the gas, because they change from winter to summer in regard to antifreeze or stabilizer additives as mandated by your government. Water is the monster in the box here in your case... that is why the alcohol is needed and will facilitate its evaporation or burning out. hope that helps!