you might attempt to use a 6E5 instead of 6U5 , they are a bit easier to find so may come cheaper. I think the green display may move a little less but someone brighter than me 🙂
Also check inside the tube socket if you find there is a resistor on the schematic that you can’t find. Usually there is a wafer type cover with holes that the tube pins protrude through. come careful work with a pick or something should pop the cover out to expose the resistor. Chances are good that this one has drifted out of tolerance. ( from about 1 meg? check schematic) I assume one could actually check the resistor before they even open the socket if they know where it is. You wouldn’t expect it to be hidden there if no one showed you.
In my experience If I can see a shadow permanently burned into them that is visible with the tube out of the radio, it often seems that those ones have sat in a radio for many years and now have a weak display. You might still see them working properly in a darkened room if this is the case. I have heard some talk that sunlight is damaging to them I haven’t made a point of covering them when not in use, but might consider just that if they are standing in a window where the sun is on the face all the time.
Phil