In my experience, if the pistol has the square plastic fillers for the safety lever, you can retro fit a thumb safety. I have did just that MANY times. You will need the safety lever, a detent, and the spring, which you have found. It just requires you to remove the sear block from the pistol, the plastic filler tabs, and install the safety. No fitting is required.
One thing I do to my pistols in regard to the thumb safety, they move pretty easy from safe to fire, so I use a needle point file, and deepen the detent notches on the lever. This gives me a move positive feel, which I prefer. YMMV