House electric not working
Many homes of this time frame had very few individual circuits and as you have found a mix of old and new wiring. With a large section not working it may be as simple as 1 conductor at a junction has failed. Tracing this may take quite a bit of time and or special equipment. I use a Greenlee CS 8000 circuit tracer it has a self powered transmitter that you can trace the wires through the walls and will locate the failure point. These are fairly expensive but you may be able to find a used one or factory second at a reasonable price but a tool like this may be a good one to have for future problems also. The CS 8000 works on both live circuits and dead. Other than that it will take tracing the wires back and the failure point could be in a wall or enclosed space the cs8000 will pinpoint it within a few inches.