I think you'd do better with a 41MM FCR carburetor on that motor.
Tires; 18's are great for MX but 20" tires suit me well for most trail riding on my 350X. You'll just need to order a rear sprocket from Sprocket Specialists to gain some more top end. I have a 14 front and you can run a 15 but forget the case saver at 15 teeth.
Your exhaust is just OK, louad as hell though. I'd be looking for a 350X Supertrapp system with discs to add or remove......
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country