I finished wrapping up the last of the items I had on the to-do list for this radio.
I bought a Sprague “Orange Drop” 0.0068uF 1600V capacitor, and upgraded the 15kOhm resistor to one capable of 2 Watts power dissipation.
I decided to paint the front up in the same silver as the rest of the chassis, so a visit to the hardware store saw a can of “cap denotes color” spray paint- a dull military silver. Spraying it resulted in a bright aluminum finish! Not common. So, I drew an outline around the loudspeaker on card, cut that out and painted the area the speaker occupies black, as per original.
I then used some foam rubber I had to make a funnel like it would have originally had. On top of that, I fitted the Front/Front+Rear/Rear loudspeaker selector switch that came with the other radio once I discovered where it attached to the radio. That has now given the option to install another loudspeaker in the car (potentially in the back, or another one with higher fidelity characteristics).