Jen, I have used a chisel to remove mortar from tiles. Lay them on a flat surface, take a wood chisel and scrape the thin set off. You can use the edge of the chisel to score the mortar and then the flat side to scrape it off. It takes time and you have to be careful but it can be done. I've tried wire brushing but it just seems to make the mortar slicker. Your palms will love you for this! Another solution would be to use contrasting tiles. You could remove some of the good tiles and use contrasting tiles to design a pattern into whatever you're repairing. Good Luck.