Hard start is more than likely a valve train issue , I build quite a few four strokes now so I have descent experience with 450s . You said it was a big bore rite? Does it have cam mod done ? Anyways my best results always include a bored carb, adjustable leak jet, ncvq needle , 48 pilot 175 ish main . if you live semi close I can look at it and do a leakdown , compression and go through valve train and tensioner system . Something tells me it wasn't assembled correctly