That's the best solution (IMO). The K&K pure western mini is a very natural sounding pup, and with the vitage jack, there is no mod to devalue your guitar. 3 little transducers glued to the sound board can be easily reversed without any problem.
First, you did not say whether the guitar was strung with nylon or steel but Martins made before maybe 1926 or 1927 are not braced for steel.
In any case, there is no real difference in the pickups for both types of guitar. I like Highlander and Sunrise pickups. Not sure how they are attached but replacing the end pin in any guitar is not going to really hurt its value.