Keep to original value because the time constant of that negative rail plays an important part in how quickly the relay drops out on power off. Also don't forget just how long this must have worked for since new... so just fit a good quality modern replacement.
Ultimately the 'ripple current rating' is an important factor (given in data sheets for caps) and higher is better. Also (and I'm not saying to do this) but fitting something like a 4.7 ohm in series with the AC (in series with the diode) can dramatically reduce peak ripple current seen by the cap but as mentioned, a modern part is all you need and will likely be far better than the originals or that era.
Ultimately the 'ripple current rating' is an important factor (given in data sheets for caps) and higher is better. Also (and I'm not saying to do this) but fitting something like a 4.7 ohm in series with the AC (in series with the diode) can dramatically reduce peak ripple current seen by the cap but as mentioned, a modern part is all you need and will likely be far better than the originals or that era.