In some projects, mainly older ones, I have found resistors and potentiometers with values that are not common, like 20k and 50k. Can I use 22k or 47k instead?
In most cases, resistors and potentiometers allow a tolerance of up to 20% in applications. The same occurs with capacitors in the case of 20 nF and 50nF or uF. So, you will certainly have no problems using the standard values nearby. You can use 22k, 47k or 22 uF and 47 uF instead of 20 or 50.
