Having driven a shifter kart a number of times, I can tell you they're a blast and half...
A few things:
1) Get a flak jacket. It's a tight fitting rib protector. Trust me, you won't regret it!
2) Shift... A LOT. The faster you grab the gears, the faster you will accelerate. Remeber, it's a two stroke motor, with a power band about 1 inch wide. So keep snatching the gears faster than you think you should, and watch everything blur as you accelerate to warp speed.
3) If you're going to run it often, be prepared to rebuild the motor at least once a year. It's not hard to do, nor is it really expensive. But the top end will need work often... Less often if it's a stock build, more often if it's a race motor.
4) Did I mention the rib protector? Along those lines, if the seat doesn't fit perfectly, and I do mean PERFECTLY, get a new one that does. Your aching body will thank you. The seat needs to be quite snug. Remember, no harnesses...
5) The most compliant component of the chassis is your spine, so make sure you have a good fitting seat and a good rib protector. It's a punnishing ride, but an absolute hoot and a half when you get that thing scootin'...
Have fun!
-Josh2