The unit has 6 resistance settings -500, - 250, -100, Off, 100, 250, 500...These gauges work by measuring the voltage across a fixed resistance in series with the sender's variable resistance (look up voltage divider principle if you don't know how that works). That means that the sender has to be a resistance, not a voltage so a voltage amplifier won't work unless you can open up the instrument and add the amplifier between the fixed resistance and the gauge.
What is the recommended sender resistance for the gauge and what is the range of resistance for the engine's sender? It may be possible to add a resistor to bring the sender into the range that the gauge needs....
Not sure what the resistance is for the engines temperature sender? Maybe someone else can shed some light.