I've had very good experiences with metal fabricators and machine shops in Mexico
. I feel the least expensive and fastest way to fix this is to just have a better gooseneck and tang made locally.
My preference is for goosenecks to be ridiculously overbuilt; they have a lot of weird cyclic and shock loads, and you lose the use of your mainsail
if it breaks. So I'd have something made that is beyond anything that you'd think could possibly be necessary, and then be happy and done with this forever. I hate worrying about things like that.
For a quick hack to get you to La Paz... It's hard to say from pictures, but you could try to constrain the twisting of the mast
tang with your giant emergency
boom splicing hose clamps (or banding tool, or wire wrap tightening tool, or a string together a bunch of smaller hose clamps) around the mast to pull the tang against the mast tightly and keep it from moving. Or, stronger but more difficult would be to through bolt the tang with nuts on the inside of the mast. The overall vibe is to take some nuts to a welder, have him weld welding rods to the nuts, and then use the welding rod to position the nuts through some other hole in the mast (say where the halyards exit) and allow new, longer bolts to thread onto them. Once you tighten the bolts you then break the welding rod off the nut by twisting it back and forth. It can be pretty tricky, depending on the where the access point is, like endoscopic surgery, but would be much stronger and fix your 'stripped threads in the mast' problem. But I'd still want to just replace the whole gooseneck assembly when you get somewhere.