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I don't know much about the kids quads but doubt you can make a 50cc outrun something with an engine that's 80% bigger. That would be like my 500 outrunning a 900cc quad. I don't see it happening.
Bore and stroke are different. You can't just swap pistons and cylinders to make a 50 into a 90.If you want something to out run a 90,buy a larger machine.