I got a gearbox from Roketa. It was $160 and $40 shipping. But it worked. Sounds like you've got the same type of ATV. Here's a link to the parts detail:
http://www.roketa.com/product/parts_...onPartId=10304
It's #62.
This is the gearbox that was on mine:
And this pic shows it installed:
They are very fragile and I think your idea to weld it solid it is the way to go. Mine broke from shifting while moving it was the first day we had it out of the box (Christmas present from my son in 2007).