An update from my repair attempt:
After finding another broken part, this time a SMD resistor (open circuit) & installing all the new active components, the source now lives! It accurately outputs stable DC voltages in both ranges, so between ±100V and ±1000V.

(Neither the source nor the meter is calibrated at this moment (or were in the last 20 years), so I'm not really able to accurately judge, if it meets its specs)
I used the 2SK1412 MOSFETs from the link voltsandjolts had sent and they seem to be genuine. (Behave similar in DC and regarding their capacitances)
Attached is an image of the Electrometer measuring the output from its own voltage source.
I will now run the source for some period of time to "burn it in" and to see if it fails again.
Thanks for all of your help!
Greetings,
Simon
P.S.
What are your opinions on recapping the whole unit. (I think Keithley 2000s from this era suffer from leaking caps) Does anyone have experience with leaky caps in those electrometers or are they generally not that problematic?