From a safety viewpoint, you should replace it. If you take the valve stems out and take them to a decent plumbing supply house, you have a chance of finding a replacement. Most generic replacement handles are a waste of time since they don't usually fit properly. No clue what it would cost to replate the existing ones, my guess is that it would be cheaper to put in new ones. It is highly unlikely you would get replacement handles to fit, but you might be lucky. If you can post a picture, somebody may be able to tell.
New construction valves are pressure balanced which means that when someone flushes a toilet, you don't get scalded. I prefer a temperature controlled one which keeps the temp the same winter/summer and as the heater tank starts to empty until it then just falls as the hot water runs out (i.e., no cold is being used).