Hey I just fixed my Leader LBO-508A and thought I would just drop some part numbers here in case it helps anyone.
I replaced all the oil caps with poly caps. There's two pairs of 0.5µF/1600V caps in series, and I replaced with a single 0.22µF/3000V. These are physically larger so I mounted them behind the board, and jumpered out the second cap. I was a little worried about high-voltage isolation distances, so I floated the caps off the board about 10mm, and used some zip-ties and silicone tape to keep some distance between them. (Not sure this is necessary, I don't work with high-voltage much.)
BOM:
(2) 0.22µF/ 3000V 224PPA302KS Illinois Capacitor
(1) 0.47µF/2000V C4CAWUD3470AA1J Kemet
(2) 47µF/450V MAL204315479E3
(1) 47µF/160V MAL204211479E3
(1) 1000uF/50V Panasonic FC (I don't have the part number it was in my inventory)