My first thought was that it could be as simple as changing the mosfets, but to make sure I understood the parameters that they operated under, I tried reverse engineering the schematic.
But now I'm a bit confused. My best guess of the schematic shows that, if the resistor values are correct, it shouldn't work at all.
I see the three optocouplers.
I see three 2K resistors that I assume are the current limit resistors for the opto LEDs
I see three more 2K resistors that i would guess are in series from the opto outputs to the gates,
And then I see three more resistors of two different values. Two 75 ohm and one 200 ohm. For three identical circuits I would have expected those three parts to be the same.
But, if these last three parts are the gate-source pulldown resisors, I don't see how anything would work.
12 volts through 2K into a 200 ohm load would deliver only about one volt to the gate which is half what it would take to start to turn it on, let alone run it to saturation. For the two with 75 ohms, the gate-source voltage is only 0.43Volts.
Can you doublecheck the resistor values? Better yet, can you draw a schematic that matches the hardware?