Going to disagree with Denny here but just a little bit. You want a 60 tooth axle sprocket and a 10 tooth drive sprocket using a 420 chain, not a 35. How fast do you want to go, what will be the typical passenger weight, ( kids adults one or two ) and what kind of terrain will it used on. Your kart is large but I think it is 1"
tubeing and not excessively heavy. A 212 cc ( small block ) will get you around but to get more than 22 - 25 mph or do much serious hill climbing it would need to be hopped up and then would be a high RPM running engine and you could easily have $800 to $1000 in high performance parts. The big block engines ( 301, 420, and 459 cc ) all have bigger 1" out put shafts and require a larger and more expensive 40 series torque converter. All COMPLETE torque converter kits come with a plate that bolts to the engine and supports the supplied jack shaft and work fine. No need to construct a separate unit. If you have the money the big block would probably be the better choice. One of the
several karts I own has a 301 and the 40 series torque converter and even though it was pretty peppy it soon got frustrating having the engine top out on the governor and just stay there. Ended up hopping it up anyway just like some of the small blocks. Love it. Runs strong.