Q2C at 4.9V is a good sign, suggesting the voltage at the output of IC1 is about half the supply voltage.
You can't measure the actual value of most components in situ because other components and semiconductor junctions around them will change the apparent values. For example R1 won't measure 100k because that has D1 and part of Q1 across it which are also likely to cause it to measure differently depending on the polarity of your meter leads.
However, R7 should measure 100R as long as you have nothing connected to the L/R input. R7 is isolated from any resistive effects of IC1 by C12, so 21R means something is wrong.
IC1 pin five is pin 5 of the LM386 chip. Just look up the LM386 data sheet, but the image is
here
In case you haven't found the circuit diagram of the MiniMod it's in the original article link below. Remember I changed Q1 to a BF256 which has different pinout to the original 2N3819.
Other than very carefully checking the values of each component - by lifting on end if necessary to remove it from the effect of other components - I'm not sure what else to suggest, except you posting a list of voltage measurements from the negative battery terminal to all the active device terminals.
Unfortunately during this lock down, I'm not in Fakenham where my workshop is and I don't have a MiniMod to hand to exactly compare voltages, but I should be able to suggest what's wrong from your list of voltage measurements.
Ian