Nippon Chemi-chon capacitors have excellent reputation for durability. Despite this, the particular dielectric technology can have a limited lifespan regardless of the brand.
I hate replacing parts without need, but it is very difficult to properly test this capacitor throughout its voltage range of 1.5kV - you never know if it has a leak that manifests itself only after a specific voltage.
All in all, if you can spare the time you can leave the capacitor, repair everything else, reassemble the unit and test it - if it works well you leave it there but if it doesn't you have a suspect.