It's always best to retain the original features of a house, and the fact that you actually like and want to keep the tiles is a plus.
But I wouldn't attempt to "fill chips and holes and just generally restore them", because all you'll end up with is a fireplace that looks as if it's had the chips and holes filled.
Replace the damaged tiles. Take a look here...
http://www.cravendunnill-jackfield.co.uk/The company is local to me and before I retired from my business in conservation joinery I used them a lot (and pointed people in their direction) and found them incredibly friendly and helpful. Drill out a damaged tile and send them a piece to compare and match.