The first thing you need to do if you want a 19m go well is check the profile accuracy, Slingsby never managed to get it as good as the Germans. That’s why the 10 gained only one L/D point after adding 2 meters. Striefeneder used to do the canopy mod, but when I asked them said it was too expensive to be worthwhile, I’ve got a photo somewhere of one that was done, and it looks great. It won’t give you any better performance though.
In the UK several have been legally modified to 20M span. A slingsby Vega canopy fits exactly, as the front was from the Kestrel. Just a canopy frame and bubble without labour will be well more than you have budgeted.