That'd be around 45mph at 10,000rpm. It'd have awesome acceleration, just wouldnt go super fast.
I've never seen any #35 counter sprockets sold for motocross engines, but you might be able to mod/build one.
#35 would probably work fine, If you dont drive like a maniac and had a high quality race chain and sprockets.
Lots of guys race 40-50hp karts with #35 chain, but most of those guys just have centrifugal or disk clutches. #420 would be a much better choice for shifter kart because your gears/shifting puts way more stress on everything...