Try Andy at PBM Racing for spares, he carries a good range of Kyosho spares and I know that he has springs in stock. 01773 769323.
In regards of mods you only really need to put about 35/40 gram of weight in the front end, atomic carbon rear shock tower (to give you the option of standing the shocks up straight) and change the ball studs links from kit, front end inner, middle hole with no washers, rear end, shortest links with 4mm of washers on the inner.
Enjoy the car.
