Do we have a list of the voltage on each gauge at various different fuel levels?

We only have "some data" to get a feel of what's going on. Each piece of data suggests more.
KNOW:
Resistance of gas tank sensor and it's direction (full vs empty)

Digital Gauge: Uses a 5V supply and likely measures current.
No point of being ratiometric.
Could just be a 5V supply and a resistor
Suspect no variation with battery voltage.
I think we have open circuit voltage (OCV) and not short-circuit-current (SSC)

OEM Gauge
Something different is going on.
Might be dependent on power supply voltage (ratiometric)

Replacement gauges and possibly OEM gauges
Do have linearizations built in.

 

A few more data points might help to make some sense of it, but I hate to think how much petrol the TS's has spilled on his garage floor in the course of this project!

 

A few more data points might help to make some sense of it, but I hate to think how much petrol the TS's has spilled on his garage floor in the course of this project!

I’d like to do those readings but bike is half apart. Can’t ride it. The fuel line is under tank, not accessible without complete removal. Though it may come to where I just have to remove it. I’ll try and figure it out

 

How about getting a few 1Watt resistors. e.g. https://www.digikey.com/en/products/filter/through-hole-resistors/53?

I searched for metal oxide 1W 11-300 ohms. Pick a few within the value of the sender, not exactly at the endpoints.

Can you measure how much current the bike draws easily. e.g. No lights and then pick a resistor that would make the bike see 9-12 Volts. Once you know the current, pick 1V and 2V drop at say 1 amp (guess). Find R from R = V/I.
then find the wattage and double it. So, get 2 resistors.

They would go in series with your battery,

Actually, you shouldn't have to do that. I think you can probably get a pigtail for fuses and use the instrument cluster circuit.

 

You could also get a few of the current source parts. You have to use the LT3092. https://www.analog.com/en/products/lt3092.html The LM334 only goes to 10mA. You would need a breakout board to mount the part.