(1) The 1N914 is
not a zener-----it is a small signal diode with virtually identical characteristics to the 1N4148.
(2) The only zener in the circuit is D7 which is used for the 5v supply.
(3)The thing is only running at 156kHz,so
ft won't come into it.
(4) In a Common Emitter stage with an unbypassed Emitter resistor,the Gain (A) is (within limits) pretty much independent of transistor characteristics,& is determined by the values of Rc & Re,so in this case,
A= R11/R12,which should give a gain of about 22 (the original schematic says 10.5---go figure!)
The 2n3904,or BC338 should work OK,as should a BC107,BC108,BC547,BC548,or even most small signal NPN transistors you might salvage fom an old PCB.
After checking your transistor looks "like a diode" for each of the two junctions,you then have to determine which leads are the Collector & Emitter.
Even if your meter doesn't have a HFE function,you can still check this,as well as whether the thing has gain.
Using your DMM in the "diode check" mode,connect the two device leads which you need to identify to the RED & BLACK DMM test leads.
Now comes the disgusting part:-
Spit on your finger and touch both the base & the transistor connection which is connected to the RED DMM test lead.
The DMM will give a reading.----write it down.
Swap the Test lead connections & repeat the test--again the DMM will give a reading.
Compare the two readings----the
higher (No! it's the lower one--higher current,lower reading) one shows which is the correct connection.
For an NPN device,the Collector will be the lead connected to the DMM RED test lead.
To check a PNP device,the polarities are simply reversed.
You have also learnt something--- transistors will work backwards,even if not very well!
The voltage rating of C5 basically depends upon what sort of circuitry you are likely to be testing.
You won't be testing active circuits,so all you normally need is a DC rated device,just in case the caps are still charged.
I think the original guy used an old 400v Poly ester cap because that was what was in his junkbox.