It might sound far-fetched, but I have successfully cleaned badly damaged PC boards in my dishwasher. I'm talking acid-damaged, and extreme dirt conditions on electric lift truck controllers and outdoor crane controllers.

Pre-spray with simple green, use the standard dish-washing detergent, let it run thru the complete cycle along with heated drying, then hit it with compressed air afterwords. This will release any junk under the chip sockets and behind the heat sinks.

You will be surprised how clean they get.

Connectors on the harnesses can be cleaned with a decent ultrasonic cleaner, using ammonia-based solvent.

If the resistors have lost their color stripes due to heat, I'd guess ALL solid state components have been de-rated and damaged due to heat. That would include all the IC's, diodes, regulators and transistors. Looks like the heat has expanded most of the capacitors, so it must've been quite intense. Fuse holder clips seem to have lost their plating, which can only be fixed by replacement.

Post some pictures of the solder side of the boards; if it got hot enough to reflow the solder, you have a lot of work in your future. Once the solder oxidizes, re-soldering becomes almost an impossibility.

I would dish-washer the boards with a double-dose of detergent and see where it goes from there.