flywheel weight is a must and keeping the big thumper cool would be the biggest problem. My buddy smoked his CRF450 motor second lap in at Unadilla. The motor was so hot his oil fill cap turned into a melted blob... It was real muddy and slow going in alot of sections. I saw quite afew 4 strokes steaming over on the side of the trail in the woods. It did rain quite a bit before the race which changed speed conditions. But even on a dry day you never know what you'll encounter at a GNCC. Don't forget larger gas tank. unless you want to pit every lap.
I would say getting the suspension right is probably one of the most important things to start with, this will allow you more comfort and will cause far less fatigue.
Moving down the list:
Consider a Rekluse auto clutch. Stalling and hot starting is one MAJOR thing that makes the bike hard to live with and causes major fatigue. The Rekluse takes care of both.
Get billet engine covers. Sounds weird I know, but they add 100-125cc extra oil capacity per side and helps with cooling and proper oiling for a full days work. Combine with good motorcycle specific full synthetic oil.
Add a radiator overflow cannister (Thumper Racing makes a trick overflow bottle or do the coolant overflow in the frame mod on the cheap)
Oversized tank and protection (watch what radiator guards you get or it will make your cooling problem worse).
Now you are very close...will probably still want some flywheel weight, but ride and assess and make your next "to do list".
I didn't mention gearing above because it tends to be pretty subjective. The Rekluse helps in this area also as it allows you to maintain gearing more towards maintaining some top end speed, while making it much better for tight twisties. Should be able to get away with +1 or 2 teeth on rear sprocket pretty easily.
My main riding buddy rides a stock 2004 CRF450R off road. Only modification is tall bars and barkbusters (he's 6'5" or 6'6") and a slightly bigger rear sprocket. He never has trouble with overheating and he is an expert so the suspension works for him off-road fine in stock form. Keep in mind we ride trails that you don't get out of first gear on.
I would recommend getting handguards, maybe a gearing change, and then take it from there.
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