I am certain that there is some serious residue and really bad gas left in the cell. IF I can clean this out then I would consider having the engine repaired.
First, a little reminder about safety. When working with combustibles such as gasoline, make sure you have a water source and a fire extinguisher on hand. It's also advisable not to work alone, in case of accidents. If your tank has a lot of sediment, the only way to properly clean it is to remove the tank, remove the sender/fuel pickup flange from the tank and pressure wash the tank. There should be an inspection deck plate located in the floor, directly above the sender flange. Remove the flange and have a good look to determine the amount of crud in your tank. You will have to cut part of the floor out to remove the tank, and then replace the floor when the tank is re-installed. If the tank is an older steel or aluminium tank, you may want to replace it with a new, polyethylene tank. Lots of sizes and shapes available from any marine supplier. The cheap fix is to disconnect the original tank, and use 6 gallon portable tanks to run your boat. The portable tanks will take up a bit of room under the back seat, but will save you $100s, and a lot of time.
you may be attacking you problem backwards.is it an i/o or outboard?get the engine running on a test tank first,and see if it's even worth fixing.if it's an i/o you may have issues with manifolds/risers,boots/bellows,etc. if it's an outboard the entire fuel system will need to be cleaned,including carbs. if the engine checks out,remove the anti siphon valve at the fuel tank and replace it with a hose barb,then siphon as much fuel out as you can with the bow raised.this will get most of the fuel out.remove the fuel sender and you will be able to get a hose down to the bottom of the tank to get the remaining fuel out.it helps if you jack up one side of the trailer and raise the bow so the fuel will run to one corner of the tank if it's on a trailer.it wouldn't hurt to put 5 gallons of fresh fuel in and repeat the process.check the fuel sender and replace with a new one if needed.install a water separating fuel filter if not already equipped.be careful and have a fire extingusher close by.even 10 year old fuel will blow your a$$ up. don't forget to check the anti siphon valve and replace if needed.
