The curse of the Agusta Bell situation...
Is this G-SPEY? Why can you not rebuild the old 'box?
Can't offer much help but I recall that Helispares had my old gearbox to part out years ago, they may still have the case. I cannot recall what the p/n was though, it may be too old. But if not, you could buy that and fill it with new gears etc. Won't be cheap though.