Anyone in your area that does aluminium welding will be able to repair that but the problem will be damaging the seals in the gearbox.
I have one that had most of the cavitation plate snapped off one side. Bought a piece of 10mm sheet aluminium on ebay, shaped it to fit and got it welded by a firm that repairs cylinder heads for £40. Bit of work with the angle grinder, some flap wheels & a bit of paint it's invisible.
Unless you're able to keep it cool to stop heat damaging the internals you might have to strip the gearbox before you start the repair which could be the hard part !!
Good luck with it.