Sadatwins,
The main issue you have initialy is adjusting the bias points in the circuit. This may be a significant issue in most of the stages as they are "self" or "current" biased thus the resistors from the supply rails to the device bases will almost certainly need to be adjusted.
The first thing you have to remember is that Vbe of silicon transistors is ~0.7V at room temprature which is half your supply voltage which is going to cause you a voltage overhead issue and the circuit shows the bias points as being outside this voltage range. Which as germanium devices are now quite rare why many very low voltage circuits are FET based.
The circuits you show have four basic sections.
1, Self Oscillating mixer.
2, Multi stage IF strip with AGC
3, Combined audio detector and AGU control generator
4, Audio amplifier.
Once the bias points have been fixed the next issue is where if anywhere is any extra gain needed?
The first stage consists of a self oscilating mixer with the antenna being an "on frequency" tuned stage with the ferite rod core being the method the RF field is coupled into the receiver. Thus the gain can simply be improved by increasing the size of the feritte rod or finding some way of increasing the field it couples into. You could simply add an idler turn around the rod that is connected to a long wire antenna or other kind of RF amplifier and antenna.
You could also improve the gain by playing with the tap point of the transistor on the collector tuned circuit which is tuned to the IF frequency. You will probably need to adjust the number of turns on this inductor anyway due to the 50% reduction in supply voltage to ensure you get the device to socilate at the emmiter tank frequency reliably.
The way to get this stage reliable is to first disconnect the input tuned circuit by connecting the base directly to the bias resistor, then replace the IF tank circuit with an appropriate load resistance that supplies the same current into the collector LO feed back coil to the emmiter LO tank.
You then replace the load with the IF tank circuit. You can increase the gain of this IF tank circuit by changing the tap point on the IF tank circuit coil or by changing the ratio of the turns from the IF tank to the coupling loop to the next stage.
I would, as the IF strip has an AGC loop from the detector not look to change this circuit unless you have to. The reason for this is the diode detector and lowpass filter used to recover the audio also supply the AGC DC feedback voltage that changes the bias point thus gain of the first device in the IF strip.
The AGC voltage is subtractive that is the detector diode produces a negative voltage that is used to supply voltage bias to the first IF transistor. As the signal level increases an increasingly negative voltage takes the bias voltage of the first IF transistor down effectivly turning it off via the lowpass filter formed by the 2k2 resistor and electrolytic cap down to ground.
The simplest way to add gain is to either change the turns ratio on the audio output transformer, and/or to change how the audio transformer primary is driven. If you change this to a bridge driver then you will get double the voltage swing and for times the power into the primary turn.