You could find a clutch for sale in Germany at this search link:-->
http://www.google.com/search?source...1T4GZAZ_enUS369US369&q=Go+kart+parts++Germany
6:1 gearing is a good suggestion, especially if you use one of the lower-cost/lower-quality, centrifugal clutch brands or models. But If you buy one of the (higher-quality) racing clutch models, your clutch could withstand gearing your kart as low as a 4:1 ratio, with 11" rear tires. (or possibly around 5:1 with 13" tires)
But when using these gear ratios for higher top speeds, you'll want to immediately get the kart up to a fast enough speed so the clutch is no longer slipping. This means you'll need to only ride the kart in larger areas without any steep hills (or rough terrain/or any other situations ) that would require the clutch to be constantly slipping.
When you start riding the kart, you'll be able to feel/hear the clutch slipping when you take off from a stop. Then once the engine reaches a certain/higher-RPM, you'll hear/feel the clutch fully engage/lock-in. (at that point the clutch doesnt have any slippage/friction)
So when gearing a kart for higher top speeds, you'll simply need to limit the amount of time the clutch is slipping, because thats the only time the clutch is building up heat from friction, and this heat will eventually damage any clutch. And gearing for higher top speeds, compounds this problem...
