If you are familiar with Arduinos and can write some simple code in C, reading the gauge, then PWM a light coupled to a photoresistor, you could program it to track exactly, even if your tank has irregular shape (as in the fuel level isn't directly proportional to fuel height).
If it was reversed and the sender was 90K and the gauge 56K you could parallel a 120K or so and not be too far off in the middle, but still have full and empty read correctly. Some senders are even reversed.
Do they both go to 0 ohms for full? Maybe with a 100K biasing resistor to 12V you can make it work well enough.