I agree with others who have said there is a need. It's a niche market, but so is virtually any camera repair currently. You see somewhat narrowly focused (I don't mean that in a negative way) specialists with a national, if not international, market.
One caveat however. Unless you are rebuilding with exactly the same cells, there might be characteristics beyond volts and mah (that's the extent of my knowledge) that need to be considered. I suspect it's not an issue with electromechanical items like a drill, but could be an issue with electronics. So some testing on your own equipment might be necessary.
For example, I know someone (OK - it was me) who fried an electronic flash by using AA nicads instead of AA Alkalines.