It's prob easier to replace the box, and there's a wide choice, the W5x series Toyota ones are OK. BTW how the heck do you bust 5th? That's quite hard to do, compared to lower gears. I presume you have a box with a bolt-on 5th gear section, it's still very difficult to do in situ, so you may as well change the box or repair it properly. Wreckers usually won't separate individual gears, IME.