I cannot get the stock fin data off the archived 2006 website as it is loading too slowly and my browser is timing out.
What size fin(s) do you have now?
As I remember the S-Type 126 came with a slalom fin and a looser wave/bump and jump fin.
The slalom fin was OK, but I think for a 7.5 m2 (not 8.5 m2).
I would look for a fin of similar design but 3-6 cm longer in span.
Let me know what fins you have and I will try to be more specific.
What brands of aftermarket fins are you considering?
Hope this helps,