These are the implementations on the asset store I’ve been looking into:
Box2D for Unity
Farseer for Unity
Because of the way Unity works (and frankly if it didn’t work this way that would be very bad), all scripts compile down to the same thing. It doesn’t matter if you write
var integer:int; or
public int integer;, after compiling it they are one and the same.
This allows for cross use of each language. You can always (except in very specific cases) call any function in C# as you can in UnityScript. Sometimes the documentation of packages are only in C# (and rightly so), but translating it is quite easy.
From a technical point of view, the only real difference between UnityScript and C# are syntax and defaults (as well as missing feature implementations in UnityScript)
Hope this helps,