You can build a Citroën 2CV from scratch with entirely new parts - almost - because someone somewhere makes what you need.
About the only mechanical component you can't buy is the gearbox "bucket", presumably because nobody ever breaks those. You can actually get various gearbox upgrades for them which is worth it if you daily one, because the mainshaft nut can slacken off and get you jammed in gear.
> get you jammed in gear
Terrifying prospect. What are your options in that situation? Would turning off the ignition have any effect?
Just stomp the clutch and shut it off
Press the clutch, brake to a stop.
Unscrew the bolts holding the top of the gearbox where the selector mechanism goes, lever the main gear back into place with a long screwdriver, and then use the screwdriver to tap the mainshaft nut around until it's reasonably tight again. Then drive it home where you can fix it properly.
At least, that's what I did.
Ah not so bad then. I thought "stuck in gear" meant the clutch wouldn't allow degearing.