I went at the broken part with my calipers, and modeled it up in Fusion 360.

I shared it on Fusion 360 so you can copy it.

http://a360.co/2DL1veh

Or you can simply download the STL files from my blog post and 3d-print them directly.

https://lukecyca.com/2017/olfa-knife-repair.html

Once you've 3d printed them, jam a socket-cap M3 bolt into the knob, and a nut into the shuttle. If you heat them up a little, the plastic melts and gives way a bit, resulting in a nice friction fit. You could probably use a bit of epoxy or superglue for extra permanence too.

All fixed!