I put that very engine on my kart. 420cc Predator with electric start. My kart is a two seater thunderkart, live axle. As stated, the engine base does have a larger foot print. For me this wasn't too much of a problem. I have a box-frame that the jackshaft runs through, and the engine mounts on top of that box. Mine is big enough that all I had to do was drill new holes.
"Gokarts4Life" is spot on about the torque converter. Follow that link and that is the exact torque converter that I now use. You will need a jack shaft for this set up. Also don't forget that the battery does not come with the engine. That is another ~$40.
The engine is already set up with a clamp for a throttle cable so it makes that connection very easy. You will have to add a spring though for throttle return. I also turned down the idle. You will need to check for clearance as well. This motor is BIG. My frame used to have a 5hp Briggs & Strat, this 420 takes up a lot more space than that little 5hp.
I also very easily moved the keyed ignition to the steering column support. A kill switch still might be a safer option, but at least I can still turn off the engine from the driver's seat.
Please let me know if you have any specific questions or would like pictures of specific areas on the engine.
I did take a very short test drive yesterday before pulling the rims off the kart; New tires are my next step; here is my kart:
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