C1 has a schematic value of 200µF and they used a 290µF in the instrument, which is fine since as you said this is for smoothing the signal across the meter, and that calls for a non-critical value, unlike in a tuned circuit.
Any cap rated at 600V can be replaced by a 630V rated replacement. Anything above 600V, but not below, and that is true of all capacitors.
1000V caps are readily available – I Googled 0.01µF 1000V and got lots of hits. You’ll be surprised to see how now small these are compared to the original!