When it happened to us, we took the batteries to a specialist (at the Airport I think) to get them professionally re-charged.
Very....very wise move.
The main attractions of NiCad was the physical size and voltage retention when under load, - not an issue now clearly, but the main down sides were maintenance costs and potential hazard. The hazard being reduced by
regular quality (and no doubt expensive) maintenance.
In service IIRC they were changed every 28 days or so (I think???) and even unused on the shelf over that period you may see a 10-15% reduction in output. In short, regular maintenance is critical to both performance and
safety.
Some form of mains supply would be ideal for the longer term, and I don't see why a set of Lead Acids wouldn't suffice as a standby in the near term, the difficulty will be deciding when to stop using the NiCads bearing in mind their potential for runaway will increase as they degrade.
"Professional advice"...again...wise move.
Oilcan