Craig at NOSValves is kind of the man on these.
I've rebuilt a few- they're not that outrageously difficult, but you do need to make sure that you take care in having the right documentation for the right revision you have (they did make some on-going production changes quite often, during the years they built these amps). I've spent some considerable time during working on the ones I've had here, just making notes and verifying what's in the amp, before actually starting the work itself. Getting the right values in there is important, as is maintaining the unit in as original of a configuration as possible (mods are generally not a good idea, IMHO).
Regards,
Gordon.