You need to have a MK3 or MK5 Escort gearbox end casing or full gearbox for starters. You also have to to drill 2 new holes in the front panel and one new hole in the OSF chassis rail as not all the holes from the origional S2 ARB mounts line up. These 3 holes will also need metal tubes inserted between the tops and bottoms of the front panel to stop the panel crushing when the bolts are tightened as they will work loose otherwise.

Not a hard job really and very worth it.