What should the minimum voltage rating of replacement power resistors be?
I wouldn't worry about this too much.  The implicit minimum voltage rating of a resistor is

&nbsp;&nbsp;V = sqrt(W &Omega;)

where V is the voltage, W the power in Watts, and &Omega; the resistance in Ohms.  If it were anything less, then the resistor couldn't dissipate the rated power.

The lowest of your choices is the 5 W 1 k&Omega; resistor, which must be able to handle at least 71 V to dissipate its rated 5 W.

This circuit has &plusmn;81 V supplies, with four of these resistors in series from one supply to the other.  Assume worst case that the center point can be driven to either rail, so 162 V across two resistors in series.  It looks like the resistors will divide that fairly evenly, so a bit over 80 V across any one of them worst case.

Any resistor you find that can dissipate 5 W or more is going to be physically big enough to handle well over 100 V.  You're not going to be able to find a resistor that meets the power and resistance requirements that can't handle whatever voltage this circuit can possibly throw at it.