Re: fuel gauge for bike
The type of brass elbow you're talking about came on some of the outboard motor gas tanks. A marine supply might have them or if you frequent the landfill as I do, then you may find discarded fuel tanks. When they get old they sometimes leak and end up getting tossed. Also, some of them had a separate gauge unit with a float inside. If you were making a tank from scratch one of those gauges would work.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...