As to the fuel gauge, several outfits market aftermarket gauges, but a couple things:
First, you must make certain fuel sender is working correctly. Empty the tank and the sender wire on an ohmeter should show about 74 ohms or more when clear empty (See figures in yellow in photo)
You can estimate tank 1/2 full which might be "a guess" and sender should show around 23 ohms, and when full, around 10 ohms
BE AWARE that different gauges react to different senders from different OEM tanks. So research the guage you buy (Autometer, whoever) and get one that covers the ohms range shown.
Also be aware that some gauges have THREE electrical terminals. Sometimes the 3rd terminal is not obvious, because it may be via the gauge metal case. That terminal MUST be grounded. Those types gauges are called a "bridge" circuit.
Also, there is a device called "meter match" which is installed between the gauge and sender. It scales the sender output to cause the gauge to read more accurately. You program it to "be so."
TechnoVersions - MeterMatch for Analog Gauge Correction
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What is the manufacturer and model of the fuel gauge you have now?