There are two types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC)
With the NTC thermistor, when the fuel level drops below it the resistance decreases, with the PTC thermistor, when the fuel level drops below it the resistance increases.
The Early FZS600s most likely had the NTC, but I do not know what type is in the R1
See picture of the fuel senders.
Looking again at the Fuel sender of the 1000 you probably could modifiy it to take the R1 sende by removing the electronic board and desoldering from the connectors and attach the R1 unit to it and solder the wire to the appropriate connector. Make up a link from the stand to the second wire connector
With the NTC thermistor, when the fuel level drops below it the resistance decreases, with the PTC thermistor, when the fuel level drops below it the resistance increases.
The Early FZS600s most likely had the NTC, but I do not know what type is in the R1
See picture of the fuel senders.
Looking again at the Fuel sender of the 1000 you probably could modifiy it to take the R1 sende by removing the electronic board and desoldering from the connectors and attach the R1 unit to it and solder the wire to the appropriate connector. Make up a link from the stand to the second wire connector