Well if you've got the tools it shouldn't be terribly difficult to reweld the receiver... just go slow and be careful because it's easy to bugger the bolt. If you follow the general rules of rebuilds and the advice of people out on the Prexis weapon builder forums you should be able to get through it without destroying your kit. Though if you've got the coin to do it... I'd still recommend sending it to Global Military Gunsmithing... having it built for you might set you back about thirty bills but you're guaranteed not to have a hiccup.
Owner, sole proprietor, and chief of cantankerousness for Sergei's Armory.