Keep in mind, that a lot of these designs were created with the "transistor du jour". Many transistors came and went over the years while some were so generically useful that they get used to replace ones that quickly became obsolete.
When looking for replacements... do a search on a site like NTE Electronics Cross Reference Search or http://www.datasheetcatalog.com/ to get the part specifications.
Then look to see what is readily available to you at the price you need.
Lets say you have a circuit that wants a 2N4124. It is basically a small signal switch that operates at 30V, and can carry 310ma load and has a GAIN (Hfe of 50-150). Now I compare my replacement. Checking my junkbox for the most COMMON NPN transistor I have... I see the 2N2222. So, will it work? Probably... are they different? SURE. But the 2N2222 is basically a 30V device and it can carry a 800ma load and has a GAIN(Hfe) of 75. Almost all these parameters are close enough for a generic replacement and will likely work unless the circuit is really depending on some odd characteristic of the original.
I actually recognize the phase shifter circuit. I bet you could use the 2SK772 from FARNELL for the N channel JFET.