Mmmmmm...odd. I agree with Rick. IIRC the ring gear is held on by, well it's a press fit kinda thing. I think once it's off, you might be screwed. I'm not sure you can just put it back on. I think welding it on may cause imbalance or warping.
Go down to your local shop and ask.
Worst case buy a new one. Or a new used one.
Of course, when you get the real answer let us know.