Just coming across this topic now.
My 3 speed Rover Rancher box also had broken teeth.
I was lucky enough to come across a 5 speed on ebay located an 1hr away.
I was also lucky in that the box came with another front guard, front suspension set up and rear guard which is good as the gearbox is larger than the 3 speed and so the rear guard gearbox hole/slot is larger.
Means I didnt need to hack up the original rear guard.
Keep an eye on ebay. Sometimes things come up.
