Hi mate, I have raced my Demon X 140 Proline,for a few years now & I run 10" Savas from a company called AF Rayspeed,they have good stocks & reasonably priced. Buying a decent rear shock is a good move,as are decent brake pads!!! If your bike doesn't come with an inner rotor kit,then it's worth getting one as it allows the engine to spin up quicker and also the flywheel under racing conditions could put undue pressure on the crank!
The DYX 140 is a great bike,and the latest one with the grey anodised bits looks the dog's!!