This week I met with the owners of the repaired instrument and showed them what I had done. They played the repaired instrument as well as several others of similar age. I was quite surprised (and pleased) that they sounded very similar. The repair does not seem to have affected the sound quailty at all.
I didn't have much time left, but I was able to drill out the plate backs. Apart from removing quite a bit of material, the holes provide a reference for the final plate thicknesses.