I don't know where on earth you'll find a new production GI type thumb safety. I have a GI piece, like new condition, parked, that I've been using as a testing placeholder in a project gun. Make me an offer by PM if you'd like.
One could also conceivably make one. It'd be expensive, but... if you look at some of the competition extended safeties (the one on newer SAs comes to mind) you'll see the back of the thumb pad is of the same general config as GI. All you'd have to do is grind off the extended portion, bevel the edges, perhaps file off the typical serrations and redo them or checker it. Whatever. It could be done, though, is my point, if you don't want to use a GI part.
As to the hammer, they're hard enough to find, too. I've seen spur hammers in Brownells... SVI makes a nice one, part no. 823-102-107.
STI makes one too, though it's a funky skelotonized piece. It's a nice hammer though, I'm using one on my aforementioned project. I also recall seeing a wide target spur hammer, but I don't recall who made it, and I can't find it on Brownell's website at the moment.
