Looks like you have a great project up and running.
My suggestions after just browsing pages 3/4 are safety based.
First, you should construct some sort of chain deflector, notice I didnt say chain guard because I dont expect you to reach behind the motor to grab the chain, but things do break, and fly off and an awesome rate which whips and can really hurt someone. It doesnt need to be much, just enough so when it does break it will be deflected down, you can make one pretty easily as they have mounting holes on the engine.
Second, didnt notice a kill switch at first, but I assume its on the steering wheel so I wont mention that. This suggestion is not safety related (was going to mention kill switch) but your recoil cover can easily be unbolted and rotated so you can pull it from the rear of the kart, it being in the front tends to snap that rope from hitting the cover a ton.
Third, your belly pan is entirely wood, I was scared on page three that you were not going to add one at all, but then I noticed you made one out of wood. Is this temporary? If not please consider welding steel under, it doesn't have to be terribly thick, but this does need to be done, wood breaks easily under the stress of our feet and with no belly pan lets get broken, very very broken.
Hope that helped some, if I think of more you'll hear about it. Good job on completing this thing once again.