Given the variety of materials available for new resistors today, as a general rule what is a good replacement for the old carbon resistors in 1930s/40s radios?

Anything.
Whatever you get will be better than what you replace.

There are some things to look for. The resistor inside the eye tube socket is always bad. It needs a 1/2 watt for proper operation.
I always replace Cand-ohm resistors because they are prone to shrt out against their cases.

All the Best,

Bill