They make repair plates that would cover the larger hole needed to replace the valve with a new one that would be both safer (federal rules require an anti-scald valve in a shower) and more reliable. Most of these are single handle. You can get 2-handle versions, but many of those are thermostatically controlled (my preference). One is the volume control, and the other controls the temperature. Single handle versions usually don't have a volume control...it is either full on or off, and the further you turn it, the hotter it gets.
I doubt a ceramic valve replacement is available, but you might be lucky...one of the pros may know. Have you checked AS website?