How to detect when current exceeds a set point
You can build a comparator and measure the current with shunt, it needs low voltage power supply, a voltage reference (zener diode) and comparator to compare voltage on shunt and reference voltage.
You didn't mention if the current is AC or DC.
I have seen in old machinery to have a current limit shutdown with relay and shunt. The resistance of relay's coil and shunt's resitance were choosen to activate a relay at certain current, not precisely but working.
You can also buy a ready made product, look for Weidmueller monitoring module PAS CMR 4,5...10 A DC or similar

I would change the control logic to 24VDC as it is an industrial standard. Then you have the option to use many industrial components. For example Weidmuller MCZ SC 0..10V has two treshold levels, you need only a shunt that can be also used to hook up an ammeter for current monitoring, or you can leave the old shunt.