It will still run at that altitude, but not well. 4th and 5th will be useless which is not a big deal because you can't go that fast on either pass and live to tell about it. The problem will be that you will have no power to climb a steep hill in any gear and there are several near the top. I went up Cinnamon and back down Engineer a year and a half ago with my TW on a 122 main jet and 13/50 gearing. I made it up the bad rutted steep section just before the top of Engineer but my buddy's TW with stock jet and gearing did not. I could only use first gear on the last 1,500 feet of either pass. You would be well advised to go down to a 118 or 120 as long as you did not try to ride below 8,000 feet before switching back. I would suggest putting a 55 on the back sprocket as well. These passes are in bad shape in places from so much ATV traffic.
You can get these jets from Yamaha and your dealer. Changing out the jet is not difficult, but you will need a JIS screwdriver to get the bowl screws out, then you can replace them with SS Allen M4 x10 bolts which makes the job much easier in the field. It's a bit easier if you take the tank off, but with Allens you don't have to. Look at the parts photos in the Write-ups. You need to hold the jet holder with one wrench while undoing the jet with another. The holder is 8mm and the jet is 7mm.
You probably should wait until next summer as both are now closed by snow.