My opinion, take it for what it is worth...
1. Assume the unit used to work correctly.
2. Assume the battery is good.
3. Assume the source and load have not changed.
A. All transistors have been replaced with 2SC828, the original parts. Assume they have been installed correctly.
B. Diodes have all been checked. Assume they were properly tested.
C. Resistors, in such low powered applications, seldom fail. Assume they are all good.
D. There are seven 1.0 uF electrolytic caps (and one 10 uF e-cap). I am not usually a fan of wholesale replacement of parts, but in this case replacing these caps may be quicker and simpler than extensive toubleshooting using test equipment the OP may not have.
Note that if the capacitor convention of showing a straight line for the + lead and a curved line for the - lead is followed, C13 is shown backwards in the schematic. C13 + should go to Q3 collector. If you order replacement caps, stay with electrolytic caps and order an extra 10 uF. Install that extra 10 uF between the +9 volt power buss and ground, with the capacitors + terminal on power and the - term on ground.
If you wish us to help troubleshoot more deeply before you replace the caps, then with the unit operating, measure the voltages on each of the 3 pins of the 4 transistors (12 measurements), annotate them on the schematic and repost it.