Depends on what you're expecting from a "drop-in beavertail safety". One that "drops-in" probably won't impress you much. I put this one from Kings on my RIA GI, and I hated it. Yes, you have to change or modify the hammer if you change the grip safety.
I had two RIA GI's, so I bought the beavertail, hammer and trigger for the RIA Tactical model from Advanced Tactical and set out to make my own Tactical. Fitting that beavertail required a LOT of grinding, filing and sanding to get it right and tight, and that required a lot of patience. Here's the mods compared to a stock RIA GI, you can see how much steel had to come off the rear "horns" on the frame.
In these "raw" photos, you can see I left a little steel on the bottom of the horns, they don't quite match the radius of the safety. I left that extra so the gunsmith that did the bluing could do the final contouring before polishing.
Then I had Novak sights installed and the gun was polished and blued.
So, yes, it can be done, and by an amatuer. Won't cost much if you finish it yourself. My bluing job was $250, but I had my heart set on that finish. So the cost can vary wildly.
Bottom line - buy a new Rock Island Armory Tactical model! You'll be much happier, and a man's just gotta have at least two 1911's!