I don't know anything specific about the Yamaha Grizzly 450 but here are a few tips. Newer carbureted toys run lean at sea level due to smog regulations. Our California XR400R dirt bikes ran fairly well up to 13,000 feet in Colorado with a few slight modifications but no jetting changes. There was a low speed fuel screw on the carburetor which I leaned out a bit. I removed an intake restrictor piece that was present. There was also a quiet core piece in the exhaust which my boys removed (I did not). Anything to increase the air flow will lean it out a bit. You might have to crack the throttle a bit to start it at high altitude. The only jet you might have to replace is the main jet. This can be easily done with a couple of tools. (A wrench to remove the big (17 mm?) bolt at the bottom of the carburetor bowl and a special little main jet tool to remove and replace the jet) You can use the altitude-jetting tables from Yamaha to find out what main jet is appropriate for your trip. I carried smaller main jets but never had to use them.
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