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The fourtrax will top out at about 55MPH.    Actually a stock fourtrax 250x runs at a top speed of 62, but with slight mods can increase the speeds to well over 75. Answer   it depends what has been done to it but a normal cr125r would go 60 to70 mph they cant go too fast because a cr450 goes 80mph to 100mph.
IIRC the Yamaha FZ400 had a redline of 14,000 rpm and was quite fast. ISTR that the two-stroke RD400 (?) also had a high redline and was fast for its time. I believe the FZ400 is about 10 mph faster than my 600 cc Seca II. My YZF-R1 has a 1 litre engine that makes 142 hp. Two years later they got 180 hp out of the same displacement. 50cc and 125cc are beginner learner bikes that can only do 30mph and about 60mph respectively. 250cc up to 400cc are lightweight bikes you have to rev hard or change gear regularly to maintain momentum but can easily do 70mph.