Put the original jets in the carb and try it - usually a carb does not need rejetted when only changing the exhaust - it's also possible you screwed up the carb messing with it and it may need a rebuild by a competent mechanic.
It's your machine so do what you want, but it should have a Keihin carb (I have no idea what a helinin hex jet is)and you do know that the round jets and the hex jets are sized differently? The round jets are sized by flow and the hex jets by thousandths of a mm - therefore a 420 round jet will flow .420 liter of fuel per minute while a 420 hex jet is .420 mm in diameter. I suggest the carb be serviced by a competent mechanic.
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