[Radioroslyn]

R8 is the tone control and R12 is the volume control. R12 is the audio gain vs R19 control the gain of the rf amp tube. Generally I find that controls don't drift in value much unless there's a is a large amount of current passing thru them. This can cause heating which in turn can cause drift in the value. This isn't the case in this application.
But wear, dirt, and cracks in the resistive strip could be a problem. This will make it intermittent. Best way the check it is to use an analog ohm meter connected from the wiper to ground side of the control. The needle on the meter should move smoothly as the full range of the control is adjusted.

9.6K or 10K seem reasonable for R20. It supplies the screen voltage for the osc section of the 6A7. I might revisit it if the there is a problem getting the osc running.

On the remote volume control ground the white. That will restore the circuit so that the rf amp will work properly. Without doing so the rf amp is disabled and the sensitivity will suffer badly.