Sigh... I just can't catch a break with this one
Everyone who has touched this car has ruined it, including the owner.
This was done by Service4Service in Surrey when they removed, bodged, and refitted the gearbox for a mere £3000. They are also the ones who filled it with gear oil. From the shape of the crack, they didn't remove one of the bellhousing bolts, and let the weight of the gearbox hang off that one bolt. They filled the crack from the outside with grey silicone to hide their crimes which means its likely too contaminated to weld.
If it weren't purple, I would scrap the car at this point and move on.
But it is purple