yes it will fit but the mounts are entirley different.

why do u need to ask? correct me if im wrong but not many s2 boxes have the lsd as standard either

i had a spare box from an mk3 xr3, which i gave to my mate to put in his S1 that was nearly 3 years ago, and its still going strong!

you best bet is to find a mk3, unless ur competent to make up a bracket!