the 100K on the kay fuzz tone is more or less the 33K R17 of the propolis
now let's get specific ... the propolis is bleeding off more signal to ground than the kay fuzz tone and lowering R17 would continue to do that. Placing a trimmer there wouldn't be a very effective volume control as there is always a signal path to the output with 0R resistance
The more traditional route would be a volume control like the fuzz face where the pot is a voltage divider going from bleeding all signal to ground (0R to ground, 500K in series with signal) to passing all signal out (500K to ground, 0R in series with signal) ... in order to do that you'd need to place such a pot on the output and remove R17
and since we're being thorough and you may get curious ... go watch a video of a kay fuzz tone if you're wondering why union tube got rid of the rocker-based tone control