As @Dud says, it depends what kind of signal you want to put in. I’m not sure I’m following what you said about the Megadrone, whether that’s the external signal you want to put in or what. But standard synth levels at least for Kosmo are usually about 10 Vpp. Most external signals will be smaller, but how much smaller depends on the source. If all you want is to shift the level then you just need an amplifier with enough gain for that source. The Mikrophonie circuit is nothing more than a basic op amp amplifier circuit:
with a gain from 5 to 130 (it says, but it seems to me the first stage is gain 6.6, not 5). If you need a different gain range that’d probably be just changing some resistors.
You might also want an AC coupled input, which involves adding a capacitor.
Again depending on what you’re putting in, you could think about doing something more complex with more functionality, such as an envelope follower in addition to the amplifier. You’ve said you just want something simple and to use up some parts, so maybe not, but as long as you’re processing external signals an envelope follower would be useful. See Envelope followers.
Barton has a guitar input module here
https://www.bartonmusicalcircuits.com/gtrin/index.html
(designed for Eurorack but no reason it can’t be Kosmo or just an external box. I haven’t built it.).