I've made 91mm Scissor springs fit the 84mm scissors fine. There are 2 areas that need slight modding.
1...The circle that compresses into the scissor tang needs a tiny bit dremeling from the end with a sanding disk, then squeezing to a tighter radius with a pair of pliers (careful, easy to crush it!).
2...the bend 1/2 way up the spring needs a tighter radius to allow the scissors to close properly/fully. Again, squeeze the spring carefully till it's right with pliers.
Takes a short while to do, but beats trying to track down an elusive spring.