The gear material would need to be investigated carefully many are high hardenability steels which are very difficult to weld without cracking and without leaving a weld with a very hard, brittle heat affected zone. Any attempts to temper back the hardness of the weld HAZ over tempers the rest of the gear and reduces its load carrying capacity. The geometry of gears in terms of the accuracy of the tooth shape and the spacing between teeth is critical and reproducing the required accuracy after welding is very challenging.
On big slow speed gears - 1 NDP teeth, 10 rpm - welds are sometimes carried out as short term fixes while a new gear is made but generally welding teeth is not an option.

I have seen this done as a temporary solution
on heavy equipment where the gearteeth were
not induction hardened.  Your gear teeth were probably
induction hardened for the hydraulic excavator.
Actually they built up the material rather than
welding a tooth and then used a template to control
a grinding operation to form the final teeth.
Very expensive.  You might be able to go back to
the original builder and have them look at the
possibility of making a new geared ring for you.  
I assume it is an internal gear.  I would be greatly
concerned about the liability of welding the teeth
in place.