I've replaced the lastin on an old WNNN, but I don't have any SS ones to look at to see if they're different.
Can you 'see' the lastin or is it sewn over in a kind of pocket?
If it's like a NN, I just unpicked the old stuff and used my sewing machine to do a 'tricot' stitch along the same place with the lastin stretch to *almost* full stretch. I've also used normal elastic when sewing other nappies and it seemed to work fine too. I think it would be pretty difficult to do by hand tho
. Sorry, probably not what you wanted to read - maybe someone else will come up with something better.