I have an old 1959 bathroom fan that doesn't work anymore. In fact it
didn't work since before I owned the house. I'd like to replace it, but I
notice that the hole for it is circular and the fan vents into the attic so
far as I can tell.

So if I slice this puppy out, am I looking at enlarging the hole and making
it square, as well as installing vent to the outside world? Or are there
fans out there that will fit the hole? It's a 10" hole right now.