You'd have to mix and match. The 7' x 7'4" wheelbase is slightly unusual but I think Alan Gibson Workshop produces a set of etched coupling rods for that wheelbase. Even if not, AGW also produces a universal set of coupling rods that you can trim and adjust to obtain 7' x 7'4". A similar, if slightly more developed, universal etch is available from Lanarkshire Model Supplies.
For the connecting rods and valve gear, AGW has about ten different types of Walschaerts valve gear etch for various British classes including the S15. The important one is that the connecting rods should come out at a scale 11' (44mm) between little end and big end centres. It's a matter of trying to work out which one needs least modification.
You may end up using more than one etch to cobble the various other rods together.