My Street Demon install experience
So this is better than ordering a new tank with a new sending unit? My gas guage doesn't work either.
Yes
What's the story about the old gas? How long has it been there? How full the tank? Through what sort of weather cycles?
Depending on the answer first thing might be to just refill with clean fuel (after draining the old) and installing a new 'sock' filter.
You may have to ask around for a bit to find place that will clean a fuel tank. Restorers and owners of old bikes, MG, Saabs and things like that, or antique cars and trucks without aftermarket tanks - those are the types of folks that are most likely to know a place in your area. Antique car shows and swaps meets would be a place to find those folks.
Fuel gage repair. Yes big time. The aftermarket ones have issues.
There's at least four or five places that will rewind the resistance wire if that's what is broken.
When you remove the sending unit, it may be obvious what needs fixing.
On the outside the wire should be attached to the sender. Don't laugh - the one on my barracuda falls off periodiclly. LOL
There also should be a ground strap from the tank nipple to the fuel line.
To remove you can make a tool for the lock ring or use a brass or aluminum roundstock or punch and gently tap ccw.
Fuel line. if its really bad, then a repop is worth it.