You will need:
40 series driven and drive unit (can get on ebay for $180 knock off or for $300+ genuine comet)
jackshaft assembly.... anyone help? (good luck finding one, I had to make mine, or modify an existing one from another kart as the block/bolt pattern is larger) The jackshaft assembly is basically an additional motor mount (see picture) that mounts to the frame on the bottom part, and the motor on the top part. It also provides the "center to center" distance that you need to know before you buy a belt.
The belt you will need to get once you find out what the center to center distance (distance between jackshaft and motor shaft) AND the driven unit diameter (typically 7 1/2")
http://www.gokartsupply.com/beltchart.htm
The jack shaft assembly also needs:
two bearings, which are usually 1 3/8 OD, 5/8" ID flanged high speed sealed.
5/8" OD solid rod, threaded on both ends with 3/16th inch keyway.
5/8" Id sprocket, w 3/16th inch keyway. Sprocket will typically be 9 tooth for most applications 40/41 pitch (you will also need to check your axle sprocket size to make sure its not #35 pitch or you could be in trouble with such a large engine)
3/16th" key for jackshaft sprocket and driven unit to be fixed to the 5/8" rod.
two 5/8 NPT nylock hex nuts to fit on each end of the jackshaft and tighten it all together.
It is also good to get a 5/8" ID PIPE (not tube) and cut it to fit over the jackshaft between the two bearings in the jackshaft assembly. This will prevent uneven wear/bearing failure when you tighten the whole thing up with the two nylock 5/8" nuts.
You may need to find someone to fabricate the jackshaft assembly for you in addition to having the parts. The jackshaft assembly just needs 3/16" plate for the structure and 1 3/8" pipe to house the jackshaft /bearings. There are also other ways of doing this, and other members may take the time to explain it in a simple way with pillow blocks, but you may run into a problem when it comes time to tighten your chain AND keep the center to center distance correct.
Overwhelmed yet? I wish i was at home so I could take a picture of what it looks like. No matter what, you will need to put in a lot of effort and money to make the engine work on the kart as it sits. You are looking at spending an additional $300-$500 to get it where it "should" be with a 40 series set up
A $100 6-7 hp predator engine and a $120 30 series knock off torque converter that just bolts onto the actual engine is probably the least expensive and most beneficial thing you could do for a go kart with decent power. I have upgraded two karts to GX390's with 40 series torque converters from 30 series 6.5 hp engines and love the extra power. I am doing it on a third now. Its really fun to drive.