yes, you'll probably need spacers/washers/bushing. i believe they can be purchased at tractor supply stores. a little tricky to find, but perfect alignment of your torque converter belt is essential to a long lasting belt! being slightly off alignment can cause premature belt wear and improper "shifting" of the torque converter.
the 212cc is going to be a bit underpowered for that big kart. i've used the 212cc, the 340cc, the 390cc, and the 420cc. $175 for the 340cc is a good deal, especially if it had electric start (you can move the keyswitch to the front of the kart). if the guy was advertising it as unused, and it didn't have oil in it - that's a GOOD thing, and pretty much guarantees that it will work unless it was defective from the factory (highly unlikely). but the comet 40 type clutches are rediculously expensive.
i will no longer buy honda/clone/briggs/tecumseh motors simply for the fact that you can get a 440cc or bigger snowmobile with a 40+ hp motor, big beefy torque converter, and electric start for $200-$500 ($500 with reverse and electric start) if you look around and are willing to take a nice little road trip. soooo much power, and soooo much fun!