Electrolytic capacitors can also dry up internally, (or leak) with age. And as they will be 30 odd years old in a power supply that works hard. I think if you've got a power supply apart, it's worth changing them on spec.
This could also cure problems with QL's that play up when expansions are added putting extra load on the power supply.
If you are going to replace them use capacitors of the same value and voltage rating as the original.
Higher voltage ratings should be OK, and for a simple PSU like the QL, within reason higher capacity ratings.