As far as the car-like shifter and tranny, I've always wondered about using the tranny and shifter out of some little front-wheel-drive car...
As I see it the hardest part is connecting the engine to the tranny - using the automobile clutch adds a lot of fabbing work, I wonder if you could use a centrifugal clutch and chain-drive it to the tranny input shaft, letting off the throttle and slip-shifting the gearbox into the next gear.
Then the second-hardest part is hooking the tranny output shafts to the rear wheels, but I'm thinking you could either put a sprocket on the left and right outputs and go with two "dead-axle" kart drive wheels, or go inside the gearbox, permanently lock the differential gears, then put a single drive sprocket to a live kart axle.
I think it's a GREAT concept, but it'd take a good deal of fabrication (and I'd think at LEAST an 8hp engine)... I think it would be cool to have an actual automotive stick-shift in a kart.
Heck, why stop there, just rob the entire driveline out of something like a Geo Metro...yeehaw!