im not good with all the brands and whatnot, but you want a 70 kit, possibly with head if you can get it, get some porting work done, a decent pipe and a suitable transmission. theres loads of guides on this site, and take a look at the scooter projects section for inspiration, i highly recommend port work. or alternatively, just buy a bigger bike, i bought a 750cc honda for less than that....
Fast, reliable, cheap. Pick any two of them, as you'll never get all three. The more that you stress a 50cc engine, the less reliable it becomes. Once you change the crank for a more substantial 12mm racing crank and add in a 70cc kit with carb kit, you may find that £600 doesn't go very far. You also need to consider if the variator, clutch and brakes need to be upgraded to handle the extra power. Scubabiker is right; if it's speed you want with good reliability, buy a 125 or bigger bike.
