well if you have 1000 euros to blow. its should go down like this F7 exhaust, open the air box lid, buy a 240 big bore kit either vitos or LA sleeve, then send that new top end to one of the technicians on this site to have it ported (before you do that decide if you want a stroker this will give more power more displacement and more throttle response) after its all ported and stuff you will need a 35mm or 36mm carb and get some new tires and a set of shocks. if you have dough left over get an extended swinger or a arms. you do all this and that 400ex wont stand a chance