I’ve got to agree with you that the FZ750 wiring diagram is not easy to trace. Being “older” has its relative meanings. Compared to any era of motorcycles before it, the bikes of the Eighties were hugely complex, and Japanese cleverness only complicated that trend. I haven’t looked at even newer bikes’ wiring diagrams to see if, like modern cars, have adopted packetized data over communication buses to simplify wiring.
Good luck with your reassembly. With any luck it all worked before it got pulled apart, and simply connecting it all up will make it work again. Cross your fingers.