What transistors can I use instead of BS170/2N7000, and what parameters to look at?
Unless you want to re analyze the transistor amplifier sections, it would be best to use the transistors specified:

Q1: is an N-channel mosfet BS170 an NPN will not work in the place of it.  

Q2: is a JFET MPF102 which is obsolete and you might want to use a substitute if you can't find one.

Q3: is a 2N5088 which is an NPN transistor, with an hFE of 300 (and it's own gain curve), while you might be able to substitute a 2N3904, the circuit won't sound the same.