You could fit it with a suitable shunt. A shunt is just a suitable resistance in parallel with the meter.
In this case if you wanted the meter to show full scale at 120A you would choose the resistance of the shunt so 50A went through the meter and 70A through the shunt. You would need to know the resistance of the meter to work out the resistance of the shunt but its not a difficult calculation. I haven't done this as I dont know the resistance of the meter but I suspect a short length of cable of manageable cross section with lugs on each end bolted to the terminals on the back of the meter would fit the bill. You'd need to rescale the meter or just rely on the deflection and OFC you'd need some fairly hefty cable to get the 120A to the dash without loosing too many volts along the way.
Maybe taking purism too far though.
Peter
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