It is fairly simple what you want to do, because you already have ballast resistors.
If those resistors drop about 0.7 volts at overload conditions, I would do the following:
-Take a general purpose PNP transistor, like the 2N3906. Call it Q11
-Connect Q11 emmiter to the junction of R21 thru R24 that comes from the positive source.
-Connect Q11 base to the junction of R21 and Q6's collector. It could also be R22/Q7, etc. Pick one pair.
-Connect Q11 collector to the junction of R30, C20 and SCR4's gate.
Whenever the current causes the voltage drop to increase above Q11's base turnon voltage, it will source current to the SCR gate, turning it on.
I built the circuit exactly as described over 35 years ago.
The only caveat: The current limit is not very precise. You have to give yourself some on the ballast resistors. Say, that they do not drop more than 0.5 volts during normal operation, to prevent nuisance tripping.
MikeMi's idea of adding an inexpensive voltmeter is a good one. Or you could use a bargraph driver like the LM3914. Back then I used an analog voltmeter.
Last piece of advice...even though you are using ballast resistors, try to beta-match your transistors.