Some updates: I not only got a wing mirror off ebay, i got it for half the price the "professional" spare parts supplier asked and as the seller lived ten kilometers away from where i was on holiday, i could pick it up in person and save shipping costs.
On the way back from said holiday, after more than 1200 faultless kilometers, i got stranded at a garage about 20 kilometers from my girlfriends hometown with the engine studdering. While i was sure the reason was moisture in the distributor cap after driving through torrential rain the day before, i lacked the philips screwdriver to remove the distributor cap (note to self: never go on holidays without your tools again), so i had to ask for professional help.
While drying up the distributor cap fixed the inginiton issue and left me with a brilliant-running motor, the mechanic and i discovered a knocking noise from the engine. The mechanic, a pimply-faced guy in his early twenties, was clearly intimidated by the Kadett's ancient engine and, after some head-scratching, said: "You'll better wait for my dad to come back from lunch. He knows such old engines."
Half an hour later, a blue-collared fat guy in his late fifties with a speech disorder showed up, let me run down what i did and did not fix or change at the engine in the last years, listened to the knocking sound, removed the V-belt, basically disconnecting both the fan and the alternator from the motor and after having the battery power the ignition for longer than i thought it could while listening carefully to the noise, it's (possible) source and how it changes over the rev range, he finally told me that judging from the fact that the noise comes from the timing gear casing, yet the engine is running just fine it can only be the timing chain tensioner. While it would be safe to drive the car home to Berlin, i should look into it ASAP. So i got myself an engine timing rebuilt kit (consisting of the seals for the timing gear casing, a new timing chain and new timing gears) and a tensioner. Will look into it tomorrow and at least replace the tensioner.
In not-entirely unrelated news i decided to pull the engine, take it apart and at least replace all seals and all bearings while i'm at it over the winter. Most likely i'll leave the replacement of the gears and chain until then, but the repair kit was only five bucks more than the seals alone.