The guage will show full, ie very extremely full if the sender or its wiring is open circuit and, presumably, empty if is shorted to chassis. I found it impossible to obtain a new sender to the same pattern but, although I cannot now find the invoice, I'm pretty sure that the one I used as a replacement on the 410 came from
https://www.smiths-instruments.co.uk/tank-senders
As you will see there is one hell of a lot of variety, also the resistance patterns vary so I'd suggest getting the Lucas part no off the original or a spares list and asking them to tell you which you need. The in tank bit wil look completely different and you need (I think -I wasnt involved at this stage) to be careful not to get the wires in the tank trapped
While you are at it I'd recommend changing the gasket, obviously, also the breather pipes, the latter regardless of whether they look ok or not.
Good luck, that seat back is a heavy old lump is it not?