Any FIRE 5 or 6 speed box will fit (although you may have to transplant the hydralics from some) -- best bet, to keep the gearing the same, is to fit a box from the same model.
Debris in the differential gears will cause this failure. When you get the new gearbox replace the input shaft bearing and seal as they are prone to failure. This will protect your new clutch and gearbox internals.