I selected one of the least-visible broken tiles to experiment on. Was able to chip out the busted tile with a hammer and chisel and only sacrificed a little of the surrounding plaster. I figure I'd rather replace plaster and grout than break some adjacent tiles so the tile replacement is going to be slow going. Mounted the new one with a good dose of tile adhesive and now will go back and grout and re-plaster around them. But it looks pretty good and I think it's worth it to replace the broken ones on the front of the station.