I agree with the people calling him a fool. I see that kind of stuff all the time. I also wouldn't want to buy that kart after seeing video evidence of them thrashing it on rough, dirty public dirts.
That said, I have a CBR600 engine I briefly thought of using for something like that, only more along the lines of a superkart. I decided against it after seeing how bad the traction and handling is even with "just" 110hp, let alone the CBR900's 150hp. Another problem is that the 600 engine by itself weighs 110lbs and makes the kart really heavy in the back end.
People still do it, though:
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Doing a side x side like this seems to improve the handling a lot.
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That is only a small selection of the videos I found.