Yoy do not have to replace the dash if it works, the 1 wire that is conected is liekly the neutral indicator sending wire.
the other wires will be the gears. the pickup contact will vary depending on the engine.
you can get a separate gear display like this for cheap
https://www.amazon.com.au/Motorcycle.../dp/B09FY4FQ4H
and a holder like this and a less cheap display that will fit into it like this..
https://www.amazon.com.au/HILABEE-In.../dp/B09J12KW4N
...or spend more and get an after market dash that has gears displayed and just wire into it that way.