This has been asked a billions of times, but yes, it can be swapped. As the RST (S2) box is knackered, I take it that the mainshaft is history. This means that you have to use the diesel mainshaft and thus the crown wheel too. Now, the diesel crown wheel can be either with 6 or 8 bolt pattern, where as the S2 has always 8 and the S1 has always 6. This means that propably you need to redrill and tap the crown wheel for the correct pattern. I've done this with mine, but I warn you, the crown wheel is not made of the softest of materials.