5 years later, replace the fuel tank or clean it?

Drain the tank and properly dispose of the waste. Most cities have hazmat sites where you can dump fuel.

Disconnect fuel line and blow out with compressed air. Replace any cracked hoses.

There can be a gooey sheet of junk in the bottom of the tank if gas was left in it and evaporated out. It can be "boiled" out. Do not leave it in, fresh fuel will loosen parts of it which are later ingested into the fuel pump and filter. The results are predictable.

The old motorcycle trick works here for a rusty tank. Add about a pound of crushed gravel to the dry gas tank. (This is the only time I've blessed the small A-bomb gas tank.) Seal the openings and shake the tank vigorously for a few minutes. Rotate the tank while shaking so the gravel can get at all of the interior of the tank. When you think you've got all of the surfaces addressed, pour the rocks out (it's hazmat, too).

Personally, I'm leery of putting a coating in a gas tank. Many motorfuel contain ethanol. Ethanol can be unfriendly to fuel system components depending on % of content in the fuel and how long it is used. If the coating isn't specifically impervious to ethanol, I'd avoid it.