More than one month has passed, but finally I have managed to get all the required components. I was still missing the 2N4125 transistors for the small VCO boards and 4 ceramic capacitors of 5pF. As replacement for latter I got some 5.1pF capacitors through some relatives in the Netherlands. The 2N4125 transistors I've found when looking for the reverb tank. Banzai Music in Berlin, Germany, who offers such reverb tanks, also sells electronic components and interestingly they also have the 2N4125 available.
Having the transistors and capacitors, I was able to finalize the three small VCO boards. Once done I've cut them apart:
And here's the reverb tank:
The other missing parts were the Peerless loudspeakers, which I've found at Straßacker Lautsprechershop in Karlsruhe, Germany: