It sounds like you may have come across the Kool Stop replacement pads. These are great at maintaining the original color and pad pattern. The problem with them is getting the originals out of the pad holder and the new ones in. I have to admit, I'm not bad with these type of things and have done them before, however, they are a pain to do. Trying to pry open the pad holders with a screw driver while keeping the pad holder stable is a challenge.
If you have a stable vise, and a piece of heavier steel that you can drill a hole into. I have a piece of "U" channel. Bolt the pad holder to the steel and clamp the steel in a vice and have at it. This can help to stabilize the pad holder while you work it out. Make sure the new pads are in there properly because your health depends on it staying put.
Or you can forget about originality and get a pad with a pad holder or an integrated pad holder. Perhaps you can save those original pad holders for a later date when you are better prepared to take on the challenge.
I like the Kool Stop Continental since it was available back in the 80's and I think the 70's. It is just bolt on. It would be considered period correct, but not original. I also like the black Jagwire basic pads. They are a pad with a pad holder and only need to be bolted on.
Someone is making a version of the 6 dot Weinmann pad in red with a holder. I was in a local bike shop for something else. 'Looked down in the case when paying for the thing that I came for and saw these and bought them. They turned out to be slightly smaller than the original, but unless you compared them side by side, you wouldn't know.
I'm sure you'll bet better advice here and maybe some better tips on how to replace the pad in the pad holder.