If the carb on the 450 has a larger venturi than the 400, it will provide for higher top end rpm but at the expense of lost low end power. To recover the lost low end power, bore the cylinder to 450cc. To complement the increased top end and low end power, change the cam, install larger valves, reduce restriction in the intake and install a larger diameter exhaust system.
I'm not sure what kind of performance increase you are looking for, but a less restrictive exhaust is the fastest and most effective "bolt on" change you can make.
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