Mr Cabot: We have PG501, 502, 506, running perfectly with all original parts, needing some switch and pot cleaning on occasion.
Carbon comp are lower self capacitance, and are still used in critical high freq and high voltage applications.
If it needs service, I suggest to replace ONLY failed/out of spec parts causing the issues, and leave all other parts alone.
Most of the changes in resistor type will make no difference.
"if it ain't broke dont fix it"
Regardless I suggest recalibration afterwards.