First thing you should do is check with you local building inspector and see if either of these old monster will pass inspection. There are some conditions that will allow old water hogs like this to be retained, but usually on renovations you must go with low flow designs. This old beasts likely use 7 gallons of water per flush and are really beyond consideration for keeping. I would suggest you look at the Toto line and select one that will meet modern code and last for many years with minimum repairs. Sorry to rain on your parade, but I really can see no sense in even attempting repairs on these things.