I've been doing some experimenting with later 316 fenders on my new 317. The fenders on the 317 look awful so I want to swap them.
The rear holes for the fenders do line up, but the front holes don't. There are a couple things you need to do to fit the fenders in.
The steps may be applicable to the 120, 140, 300, 312, 314, 316k, or 400.
Cut the footrests: Not by much, but just trim them a little bit to give full travel to the neutral pedal and brake pedals.
Add something in the gap: Either weld on a plate or add a small toolbox or something in the gap between the cutout in the fenders and dash tower. (you can see the gap in the pictures)
Move the front mounts back: Add a threaded tube, a rubber isolator, or something to lift the front of the fenders a bit and attach them to the frame.
Move the fuel tank neck: Switch the tank neck to the other side or make a new tank. This step won't apply to the 140 or 120 as they have the tank in the front.
Get a longer bolt for the hydro release, or alternatively just bend in the fenders a bit to add the hydro release.
Use this if you want the indented taillights and square edges or just have junk fenders (me, personally, I'm doing it for both reasons).