How can I find the voltage rating for a particular ceramic capacitor?
My hunch is that the part is 100nF, and your replacement is 500nF.

I wouldn't risk using a cap without a voltage rating indication on it. Also, you're better off replacing that Z5U dielectric with one that's more temperature stable, like NP0.

I'd source a 100nF, 2kV NP0 capacitor as a replacement.