Axel Gerstl has good performance parts. Though a tad far away. And expensive for europeans too i'd imagine (just check out some of the prices: yikes).
Ignition timing,spark plugs, exhaust, air filter, sports cam, performance carb, upping the compression (though more work and costs more). Don't know.
Though has anyone thought of turbocharging a 650 engine? That would be mad...