After messing around with the settings.txt file, I found that you can change the key binding.
Here is a quick workaround to bind zoom to new keys :
Edit '~/.runicgames/Torchlight/settings.txt' ( don't forget it's UTF-16 ).
At lines 47 and 48, change value 187 to 33 and value 189 to 34.
33 and 34 are key codes for 'Up page' and 'Down page' buttons.
You can find a list of keycodes here : http://www.expandinghead.net/keycode.html
There even is a JS link for a script that will tell you the code for a key you press.
This enables you to rebind the keys as you like.