John E. wrote:

What I know of the design goal of this circuit is that it must activate the
solenoid quickly from off to on and quickly from on to off with as little
"ramping" as possible. With the given circuit, what does this knowledge say
about the selection of possible replacement component(s)?

Well, there is nothing these diodes can do about the turn on
time. That is a function of the supply voltage and the coil
inductance. You would have to raise the supply voltage and
add enough series resistance to limit the steady state
current to a safe value to speed up turn on.

The rectifier cathode connects toward the positive supply
end of the solenoid, but the zener cathode points toward the
fet drain.

Anode-to-anode, with the rectifier "on top", the pair being connected across
the solenoid?

Order doesn't matter, only orientation.

Higher zener voltage means faster current ramp down. But
you will probably have to go quite a bit higher to see much
difference. The resistive drop of the coil is already
starting the ramp down with a 42 volt reverse voltage. But
that drop falls as the current falls, so the zener is really
there to speed the tail of the process, unless its initial
voltage is on the order of the supply voltage. So you might
consider one as high as 22 to 39 volts. But then I would
look for a 1 watt unit, to handle the power pulse that will
end up more there than in the coil resistance. But you
should definitely see some decrease in the power down time,
to about 37% if what you will get from a 4.7 volt zener if
you switch to a 33 volt one. So you can see that the turn
off time is not dominated by the zener till its voltage gets
near the supply voltage. But increasing the zener voltage
drop helps.