a guy at the local kart track here did one. he stretched out some old sled of a 125 chassis and used a 750cc on it. broke axles left and right. speedwise, an anderson 250cc shifter would kill it. it is actually made to handle something like that motor. at sears point (infinion) they were nearing speeds upwards of 160. with a 250. 125's are running about 130 mph in the draft and 80's are running about 105-115 in the draft. in an 80 you can smoke the tires so hardcore if you want, but it takes skill not to on the track.